L'-RARY UNIVERSITY OF CALIFORNIA DICHROISM AND SPECTRA OF PRECIOUS STONES SAPPHIRE RUBY EMERALD PERIDOT BROWN TOURMALINE BROWN TOURMALINE ANDALUSITE ALEXANDRITE ALMANDINE GARNET a B C D E b ZIRCON a B c D BOARD OF EDUCATION, SOUTH KENSINGTON, VICTORIA AND ALBERT MUSEUM. PRECIOUS STONES CONSIDERED IN THEIR SCIENTIFIC AND ARTISTIC RELATIONS WITH A CATALOGUE OF THE TOWNSHEND COLLECTION BY A. H. CHURCH, F.R.S.. M.A., D.Sc., F.S.A., Professov of Chemistry in the Royal Academy of Arts in London. NEW EDITION. LONDON : PRINTED FOR HIS MAJESTY'S STATIONERY OFFICE, BY WYMAN & SONS, LIMITED, FETTER LANE, E.C. 1905. Price Is. 6d. ; in Cloth, 2s. 3d. At Museum 1/6 EARTH SCIENCES t'BRARY CONTENTS. Page PREFACE ... v PREFACE TO FIRST EDITION . viii BIBLIOGRAPHICAL NOTES . CHAPTER I. DEFINITION OF PRECIOUS STONES . . . . . . CHAPTER II. PROPERTIES AND DISCRIMINATION OF PRECIOUS STONES. CHAPTER III. CUTTING AND FASHIONING PRECIOUS STONES . . . . . . . . 25 CHAPTER IV. ARTISTIC EMPLOYMENT OF PRECIOUS STONES CHAPTER V. ARTIFICIAL FORMATION OF PRECIOUS STONES ... . .48 CHAPTER VI. IMITATIONS OF PRECIOUS STONES . . . . . . .51 8445. 1000 Wt. 29306. 5/05. Wy. & S. 3407r a iv PRECIOUS STONES. CHAPTER VII. Page DESCRIPTIONS OF PRECIOUS STONES - . . . . . , .54 Diamond, 54 ; Corundum, Sapphire and Ruby, 61 ; Spinel, 67 ; Tur- quoise, 70 ; Topaz, 72 ; Tourmaline, 75 ; Garnet, 78 ; Peridot, 85 ; Beryl and Emerald, 87 ; Chrysoberyl, 90 ; Phenakite, 91 ; Euclase, 92 ; Zircon, 92 ; Spodumene,'96 ; Hiddenite, 96 ; Kunzite, 96 ; Opal, 97 ; Quartz, 99 ; lapis-lazuli, 102 ; lolite 103 ; Crocidolite, 104 ; Labrador- ite, 105 ; Moonstone, 105 ; Sunstone, 106 ; Obsidian, 106 ; Epidote, 106 ; Axinite, 107 ; Sphene, 107 ; Cossiterite, 107 ; Diopside, 108 ; Apophyllite, 108; Andalusite, 109; Jade and Jadeite, 110; Pyrites, 111; Haematite, 112 ; Amber, 112 ; Jet, 113; Malachite, 113; Lumachella, 114; Pearl, 114 ; Coral, 117. THE TOWNSHEND COLLECTION OF PRECIOUS STONES . . . .119 CATALOGUE OF THE TOWNSHEND COLLECTION 122 INDEX 133 ILLUSTRATIONS. FIGURES 1 to 11 . . . ... . . Coloured Frontispiece. FIGURES 12 to 20 ..." . pages 26 to 30 SPECIMENS IN THE TOWNSHEND COLLECTION : pages 122 to 125 Figures 1 to 20, Plate I ... . . . facing page 122 Figures 21 to 29, Plate II .. "^ ..... 123 Figures 30 to 42, Plate III . . 124 Figures 43 to 50, Plate IV .. 12 5 PREFACE. Since 1882, when the " Handbook of Precious Stones " was published, the volume has been several times reprinted from stereotype plates. Occasionally a few alterations and corrections were made in the text, but no opportunity occurred to improve the arrangement of the work or to add fresh material. The present issue, however, represents a thoroughly revised edition. A large number of paragraphs have been wholly rewritten, while so many additions have been made to the accounts given of the different kinds of precious stones and other beauti- ful minerals that the 112 pages of the original handbook have been increased to 140. Among the minerals which have now received fuller treatment may be named diamond, sapphire and ruby, and the different varieties of garnet and of zircon. But readers who desire to make themselves more intimately acquainted with the optical properties, the crystal-forms and intimate structure, the modes of occurrence and formation and the chemical composition and constitution of precious stones, will find it necessary to turn to works in which full details of these subjects are given. In this connection may be named the treatises of Dana, of Professor Lewis of Cambridge, Professor Maskelyne and Professor Miers, for in the pages of the present handbook only such scientific considerations find place as can be easily grasped, and which, at the same time, help to explain the beauty of precious stones and afford methods of identifying the different kinds. The chief localities where precious stones are found have leen named in Chapter VII. under the headings of the several species and varieties. But this subject cannot be adequately discussed without having recourse vi PRECIOUS STONES. to maps, both geographical and geological, to which no space could be allotted in an elementary handbook. But there is one rich district which seems to require special notice here in order to remove what seems to be a prevalent misconception. In the body of the present handbook frequent references are made to the occurrence of many gem-stones in Ceylon. The search for these beautiful minerals and the traffic in them has, in fact, been going on in that island for ages, while the plumbago and mica industries are affairs of to-day. Yet it is strange that the im- portance of the Ceylon trade in precious stones remains unrecognised not only in newspaper correspondence but in official documents and in standard looks. One meets with such a statement as this " Plum- bago is, practically, our only mineral export" ; and this, " The yield of gems in this island is not large, the total value of the annual pro- duction being said to be no more than 10,000." A glance at the true figures suffices to demonstrate the incorrectness of such statements. The value of plumbago exported from Ceylon in 1903 amounted to 119,316. Now the value of the gems exported in an average recent year by a single Colombo merchant was 30,000, while there are a score of other Ceylon gem merchants who together export no less than 200,000 worth annually. With casual sales to visitors to the island and to travelling dealers, a moderate estimate of the annual export of gems from Ceylon will be 300,000. The variety of kinds found is large, sapphires, spinels, alexandrites, chrysoberyls, beryls, topazes, catseyes, tourmalines, zircons, garnets and moonstones being the chief : diamonds, emeralds and turquoises do not occur, while pearls belong to a different category, being organic products. But it must be allowed that the precious stone industry constitutes now, as it has done for many centuries, an important feature in the resources of the island. In concluding these prefatory notes I have much pleasure in acknow- ledging the help of Dr. C. A . MacMunn, to whose skill in spectroscopy many scientists are largely indebted. He drew for me the absorption- spectra of almandine and of zircon, reproduced in the coloured frontis- piece. Although more exact in details, these drawings do not, I am glad - . , '; PREFACE. vii to say, present any obvious differences from the corresponding figures in the plate issued in 1882 with the first edition of this handbook. But they do show a marked superiority over the spectra figured in subsequent issues. The nine upper figures in the Frontispiece show the twin colours of certain precious stones, figs, i to 6 representing the hues as seen in the dichroiscope. A.H.C. KEW GARDENS, February, 1905. PREFACE. (To THE FIRST EDITION). A revised Catalogue of the Townshend Collection of Precious Stones in the South Kensington Museum originated this Handbook, in which an attempt has been made to associate, if not to combine, the scientific with the artistic study of precious stones. It has been necessary to confine this work within somewhat narrow limits, and hence to omit much which might fairly find a place in a comprehensive treatise on the subject. The writer, however, trusts that what is here offered for the consideration of students and amateurs may increase the intelligent appreciation of precious stones, and further their more judicious treatment in jewellery. Notwithstanding the exquisite skill of a few modern artist-workmen, it must be affirmed that there is room for improvement in the ordinary productions of jewellers' shops, with., respect to knowledge, taste, and finish. Chiefly in fault, however, are the purchasing public, who still tolerate the horseshoes, anchors, and clumsy cables of a debased time, and are not quick to appreciate refine- ment and originality in the selection and artistic mounting of precious stones. So a few words about these beautiful materials their nature variety, and employment -may prove of wider service than a mere descriptive catalogue of the specimens belonging to the South Kensing- ton Museum. A. H. C. KEW, Nov., 1882. BIBLIOGRAPHICAL NOTES. The following books and papers on precious stones and on related topics may be consulted, especially for such technological, historical, and archaeological details as do not fall within the scope of the present work. MDME.-DE BARRERA. " Gems and Jewels." London : Bent- ley - - 1860 MAX BAUER. " Edelsteinkunde," pp. xvi. and 712. Leip- zig : Tauchnitz - - 1896 MAX BAUER and L. J. SPENCER. " Precious Stones, " pp. XVI. and 627. London : C. Griffin and Co. - - 1904 SIR G. C. M. BIRDWOOD. " Industrial Arts of India." Vol. ii. pp. 17-32. London : Chapman and Hall - - 1881 E. BOUTAN. " Le Diamant," pp. 325. Paris : Dunod - - 1886 A. H. CHURCH. " Physical Properties of Precious Stones. " Proc. of the Geological Association, vol. v., No. 7 - - 1878 A. H. CHURCH. " Precious and Curious Stones," " Spectator," July gth - - 1870 A. H. CHURCH. " Discrimination of Precious Stones. " Journal of the Society of Arts, vol. xxix, pp. 440-446, April 8th - 1881 L. DIEULAFAIT. " Diamants et Pierres Precieuses." Paris : Hachette - 1871 C. DOELTER. '* Edelsteinkunde, " pp. viii and 260. Leipzig ; Veit 1893 2 PRECIOUS STONES. with these gems, there are doubtless several hard and beauti- ful stones which are found in less abundance, but which remain less costly because in less demand. Yet there is something to be said in favour of the high position commonly given to the diamond, the ruby, the emerald, the sapphire, and we may add the pearl and the opal : they all possess a very conspicuous and obvious beauty. By brilliancy and colour they force them- selves upon our attention, while the spinel, the jargoon, and the tourmaline generally need to be studied, to be looked into, that their merits may be discovered. But the argument that beautiful stones ought not to be employed in the higher kinds of bijouterie unless they are costly is an unworthy one. It will not bear criticism. Why should not moonstones, even if they can be bought for a shilling apiece, be introduced into goldsmiths' work of the most artistic sort ? Surely they may rank at least with coloured enamels, which are of extremely small money value, but which are prized highly when employed with skill in well -designed jewels. It has been before stated that the caprice of fashion influences and alters the market value of precious stones from time to time. The peridot, the amethyst, the cat's-eye, and the aquamarine have each had their day, and then been abandoned for new favourites. Even the emerald has suffered vicissitudes, and so has the opal. The causes of such changes in the popular esteem in which particular species of gems are held cannot often be traced. A new fashion is set or an old one restored, and once set is blindly followed. The introduction of a little-known gem, however beautiful it may be, is generally a most difficult matter. A jeweller who was in the first rank of artistic workers was showing a customer a bracelet beautifully set with the rich green garnets of Bobrovka. This lady admired the stones and the workmanship immensely, but spoke of the former as emeralds. The jeweller honestly said : " They are not emeralds, but a rare sort of garnet from the Ural DEFINITION OF PRECIOUS STONES. 3 f Mountains." Forthwith the lady rejoined : " Well, after all, I do not think I care so very, very much for this bracelet ; please show me something else." Not that she knew that there did exist a real objection to these green garnets they are not quite hard enough to stand much wear. For the ignorance that prevails about precious stones, not only among the wearers and owners of them, but also among jewellers themselves, is indeed dense. A London goldsmith had six stones to mount as rings ; in return- ing them finished, the invoice gave to the specimens five wrong designations ! A few years ago how very few jewellers understood what was wanted when a tourmaline or a jargoon was asked for ! and yet the tourmaline and the jargoon have been long known. Diamond, ruby, emerald, sapphire, pearl, opal, turquoise ; tur- quoise, opal, pearl, sapphire, emerald, ruby, diamond such is the range and variety of acknowledged gems. If a novelty has to be introduced it must be called by some modification of these well-known names, and must become a " Cape ruby " or an " Uralian emerald." In speaking further on, in reference to the artistic use of precious stones, something more will be said upon this point of the neglect of certain kinds of extremely beautiful stones. From the statements just made it will be gathered that although a stone to be precious must have, in very good measure, the qualities of beauty, durability, and rarity, yet we cannot arrange precious stones in any fixed and definite order, by assigning them places in our list in accordance with the degrees in which they possess these three qualities. Even if all stones going under the same name were equally fine this would be impossible ; much more is this the case when we learn that two specimens say of ruby each weighing the same, might be worth five pounds and fifty pounds respectively. In placing these three necessary qualities of beauty, durability, and rarity in this sequence, the intention has been to express the pre- eminent necessity for beauty in stones deserving the name of precious ; 8445, A 2 4 PRECIOUS STONES. the importance of durability, which must claim the second place ; and the desirability of a certain degree of rarity, especially where the quality of durability may not exist in the highest degree. How far a very beautiful and hard mineral would maintain its position as a precious stone in the event of its becoming exceedingly abundant, one cannot venture to judge ; but as we have to deal with existing facts only, the problem is one which practically has not yet been presented for solution. As precious stones have just to be looked at and worn, or used in decorative work, it will be readily understood why no occult property is of much moment in determining their value. Individual and learned amateurs may indeed value a stone according to what they know of its history, its romance, its memories-, or the curiousness of its components ; but in ninety-nine cases in a hundred any en- hancement of value through such causes is out of the question. Still, from the mineral ogical and chemical points of view, it is perhaps legitimate to import some elements of interest when ap- praising the right of a stone to be called precious, or its place in the list of gems. One need not follow those writers who speak of precious, semi-precious, and common stones ; but one may reasonably arrange the different kinds in a few groups or classes, according to what we may call the average sum of their merits. To assign a precise place to each species is not possible. Hence the futility of such a classification as that published in 1860 by K. E. Kluge wherein emerald takes lower rank than zircon, and precious opal comes after garnet, while to turquoise is assigned a place beneath nine other stones only one of which (peridot) is even known to dealers in precious stones, and to the purchasers of jewels. CHAPTER II. PROPERTIES AND DISCRIMINATION OF PRECIOUS STONES. SUCH properties of precious stones as are perceptible to the eye unaided by optical apparatus, but trained to keenness of vision, afford valuable means of discriminating precious stones from one another, but do not exhaust such means. Indeed, such mechanical properties as hardness and specific gravity are of the greatest use in determining the species of a stone, and are more commonly available than the majority of optical tests. Optical properties must, however, ever hold a chief place in all artistic classifications of precious stones, so that it will not be unadvisable to begin the present chapter by a synopsis of the most obvious characters of this class. They may be arranged in the form of a tabular view, the use of which is twofold, enabling us to define the several optical properties found in gem-stones, and also to appreciate their artistic capabilities. We arrange these optical (or mainly optical) qualities under the general heads of " Surface " and " Substance : " / 1. Plane. SURFACE. Form. Lustre. 2. Curved. 3. Metallic. 4. Adamantine. 5. Resinous. 6. Vitreous. 7. Waxy. 8. Pearly. 9. Silky. SUBSTANCE. PRECIOUS STONES. 10. Transparent, 11. Translucent. Light. ^ 12. Opalescent. Chatoyant. Opaque. Iridescent. 16. Monochroic. \ 17. Pleochroic. [ 18. Fluorescent. The greater number of these terms will be found illustrated in the present and succeeding chapters : we now proceed to the discussion of the qualities which underlie them, and of other im- portant physical characters of precious stones. The order which will be followed may be gathered from this scheme : REFRACTION. DISPERSION. POLARIZATION. PLEOCHROISM. HARDNESS. SPECIFIC GRAVITY. FORM. STRUCTURE. Refraction of Light. The familiar experiment of plunging a stick in a vessel of water and observing the broken appearance which it assumes, serves to illustrate the action called " refrac- tion," or bending back. This refraction of light occurs in the majority of cases where a ray of light falls upon one transparent medium from another say from the air upon a diamond. Part of the incident light enters the diamond, and follows a different path is refracted. The diamond, like liquids, glass, and other molten or vitreous that is non-crystalline matter, possesses the property of simple refraction ; many precious stones, indeed the REFRACTION OF LIGHT. 7 majority, are doubly refractive. A bright spot of light, say a small candle-flame, when viewed through a single refracting stone appears single ; through a doubly refracting stone, double. The stone should be moved from the eye until, even when at a con- siderable distance, the flame seen through it appears single or double, as the case may be. All crystals belonging to the cubical system, such as diamond, spinel, and garnet, are, like glass and strass, simply refracting ; ruby, beryl, topaz, and quartz are all doubly refracting. There are very precise and beautiful methods for ascertaining this quality in transparent crystals, but they are not applicable generally to cut and polished gem-stones. The results of some of these accurate measurements of the indices of refraction of transparent minerals will be found in chapter vii. ; that they differ much in different species .may be seen from this brief list of indices for the yellow ray : Diamond . . . 2-417 Zircon .... 1 '950 Almandine . . . T810 Ruby . . . .. 1-779 Pyrope . . . T750 Chrysoberyl . . 1'748 Peridot . 1-659 Tourmaline ... 1 -642 Heavy flint glass . 1 -619 Beryl . . . . 1'575 Rock crystal . . 1 -549 lolite . . . . 1-540 Crown glass . .1 '524 Water 1-336 Although this series of refractive indices may be accepted as containing numbers near the truth it must be remarked that every doubly refracting substance has two indices of refraction for each ray, although the difference between these indices rarely, if ever, exceeds five units in the second decimal place, and generally amounts to no more than one unit. And it should be mentioned that the same species of stone, even in its apparently purest con- dition, does not present, in all specimens, precisely the same optical features ; there are differences due in part to chemical, in part to molecular causes. Thus there have been observed in fine diamonds variations of refractive index amounting to several units 8 PRECIOUS STONES. in the second decimal place, while in the case of zircon specimens of the different varieties have furnished, for the ordinary refractive index with yellow light, figures ranging between 1-95 and 1*84, that is a difference of 0*11. The unit to which all these indices are referred is that of air, which is taken as i ' o. In connection with the refraction of light by precious stones mention should be made of the phenomenon of total internal reflection. This, so con- spicuous in the case of diamond and other gems of high refrangibility, fills the stone with light and contributes very largely to the beauty of its appearance. This subject is fully discussed in Part I. of the volume on Precious Stones by Dr. Max Bauer and Mr. L. J. Spencer. Dispersion of Light. When a ray of light passing from one medium to another is bent or refracted, the light being composite and consisting of rays having different degrees of refrangibility, it suffers dispersion as well as refraction. In this way the several component rays, differently coloured, are separated more or less widely from each other, .and are said to be dispersed. Upon this property of gems depends that peculiar quality of " fire " the play of prismatic hues, which is the most marked characteristic of the diamond. It is the difference between the extreme indices of refraction of the red ray and of the violet ray at the ends of the visible spectrum. It is best measured by taking as standards certain fixed lines in the solar spectrum ; but for the purpose of comparing the dispersive powers of different stones the following list of approximate dispersion- coefficients (quoted fromM. Jannettaz) will perhaps suffice : 027 019 015 014 Polarization of Light. There are several ways in which light may suffer the remarkable change known as polarization. If we Diamond Borosilicate of lead Flint glass . Blue sapphire 044 049 .036 029 Red garnet Tourmaline Crown glass Rock crystal PLEOCHROISM. 9 assume that a beam of ordinary or natural light*, freely traversing any medium, has what we may call identical properties on all its sides, then, should that beam encounter any obstacle, as by reflection or refraction, it exhibits to a greater or less extent different properties on different sides is, in fact, polarized. One quality of this polarized light is that it cannot be again reflected at a certain angle, nor can it again traverse in a certain direction the crystal in which it has suffered this change. But the amateur of precious stones is mainly concerned with these two facts, that in some doubly refracting crystalline minerals the two oppositely polarized beams are of different colours ; and, secondly, that some transparent gem-stones are more or less opaque, in one direction at least, to one of the two oppositely polarized beams. Thus it will be clear that upon double refraction and its concomitant polarization depends that property of many gems which is known as pleochroism, and which may be mostly easily recognised by that useful little instrument, the dichroiscope. Pleochroism. When a distinctly coloured precious stone is examined by means of a dichroiscope it will invariably show two images of the same hue or of different hues. Should the two images of the square opening of the instrument be identical in colour," then the specimen may be a garnet, a spinel, or a diamond ; it cannot be a ruby, a topaz, or a beryl, all of which show twin colours differing in a perfectly recognizable degree from each other. However, before proceeding with the description of the special applications of the dichroiscope, a word on the construction of the instrument may be introduced. It consists of a cleavage rhombohedron of Iceland spar, having its longer edges nearly an inch long and its shorter edges about three tenths of an inch each. In the original form of the instrument a small glass prism of 1 8 was cemented on each of the small end faces of the prism ; but this may be done away with if these end faces be ground and polished so as to be perpendicular to the length of the prism. 10 PRECIOUS STONES. A sliding cap at one end has a square perforation of about *I2 inch ; at the other end is a lens, or combination of lenses, of such focal length as to show a distinct image of the square opening when the cap is pulled out a quarter of an inch or so. With an instru- ment so constructed the pleochroism of the vast majority of gem- stones may be determined at a glance. Of course this quality is so conspicuous in some species (tourmaline and iolite) that no instrument is usually needed to discern it. For it is easy to notice that the colours of some crystals, seen by transmitted light, vary with the direction in which they are viewed. If the transmitted ray be analysed by a Nicol's prism, its colour will be found to vary as the prism is turned round its axis ; in fact, the two differently coloured beams are polarized in opposite planes. It is of course only in doubly refracting crystals that this phenomenon of di- chroism occurs. In the descriptions given further on, of the several species of stone, these twin colours, as seen by the dichroiscope, are duly recorded. Here, however, it may be useful to group a few conspicuous instances of dichroism together ; several are illustrated by Figs, i to 9 of the Frontispiece. NAME OF STONE. TWIN COLOURS. Sapphire (blue) . . Greenish straw Blue. Ruby (red) . ... Aurora red . Carmine red. Tourmaline (red) Salmon . Rose pink. ,, (brownish red) Umber brown . . Columbine red. (brown) . Orange brown Greenish yellow. (green) . Pistachio green . Bluish green. (blue) . Greenish grey Indigo blue. Emerald (green) Yellowish green . Bluish green. Topaz (sherry) .... Straw yellow Rose pink. Peridot (pistachio) . Brown yellow Sea green. Aquamarine (sea green) . Straw white Gray blue. Beryl (pale blue) Sea green Azure. Chrysoberyl (yellow) Golden brown Greenish yellow. Iolite (lavender) , . Pale buff . . Indigo blue. Amethyst (purple) . . . Reddish purple Bluish purple. HARDNESS. ii When examining a stone for dichroism it is necessary that the specimen should be looked through in some direction other than that (along a certain optic axis) in which the crystal is only singly refracting. Trials in different positions, for the optimum effect, having been made, the stone should be fixed in such a way that it can be placed close to the square opening of the dichroiscope. A disc of millboard having a hole in the middle may be used as a holder, the specimen being fixed in position by means of a little blackened beeswax. Then the two images of the square opening of the dichroiscope should be focussed sharply by means of the sliding cap. It will be observed that one image of the opening is nearly central ; this represents the ordinary ray, for which, in the present volum , the symbol o> is employed. The other image, formed by the extraordinary ray and expressed by the symbol e, is more displaced, and is distinguished by a narrow blue border on its outer edge, and by a narrow red border on its inner edge. On turning the instrument round the greatest differences of hue between the two images furnished by a dichroic stone will be seen four times during a single rotation ; four times the two squares will be identical in colour^ These phenomena correspond to eight positions, all 45 degrees apart in the circle of 360 degrees. With coloured glasses and all other singly-refracting substances the images are alike in all positions. Further discussion of the optical properties of precious stones, including the colour-effects produced by diffraction, absorption, and interference, would be out of place in a handbook of elementary character. For detailed description of the phenomena in question reference may be made to any treatise on experimental optics ; for a brief account the author's little book on " Colour " (Cassell & Co.) may be consulted. Hardness. One of the characters by which gem-stones. may be distinguished from each other and from their^imitations is that of the degree in which they possess the power of resisting abrasion, i2 PRECIOUS STONES. Many hard minerals may, however, be easily broken, fractured, or chipped, though they cannot be scratched : a very hard stone may be a very fragile one. Emeralds, zircons, and diamonds have often been ruined by a fall or a blow. The scale of hardness adopted for minerals was devised by Mohs. Fragments of transparent minerals, which may be conveniently mounted in handles, are applied in succession to the stone under examination, so as to attempt to scratch its surface. When the stone neither scratches nor is scratched by any member of the scale, the hardness of the two stones is the same. When it scratches the softer, and is scratched by the harder of the two test-stones, some notion of its position between them may be gained by passing all three specimens, with slight pressure, over the surface of a fine, clean, hard file, one end of which rests upon the table, and noting their different degrees of resistance to abrasion and the sounds produced. In chapter vii. of this book will be found, under the description of each kind of precious stone, numbers which nearly represent the average hardness of good specimens of the several sorts according to the common mineralogical scale, which is Diamond . . . .10 Sapphire .... 9 Topaz . ... .8 Quartz .... 7 Apatite . '.. ' V . 5 Fluorspar . . . . 4 Calcite .... 3 Rock Salt or Gypsum . 2 Felspar . ... 61 Talc . ; . . , . . 1 A list of the degrees of hardness of a considerable number of different gem-stones will serve to show their relative positions with regard to this scale. Although this character of hardness cannot be extensively used in the discrimination, of cut and polished gem stones, yet it is sometimes available even in the case of such speci- mens when unmounted, the " girdle " of the stone offering a suitable surface for a trial of hardness. Diamond . Sapphire and Ruby . Chrysoberyl Spinel Topaz . . Aquamarine . . . 10-0 . 9-0 . 8-5 . 8-0 . 8-0 . 7'8 lolite ,, . Cinnamon stone Jadeite . Amethyst . . . Peridot .... . Jade . Emerald . Zircon Tourmaline Phenakite Almandine . 7-6 . 7-5 . 7-5 . 7'5 . 7'5 Moonstone Green garnet . Turquoise . . . Opal Lapis-lazuli SPECIFIC GRAVITY. 13 TABLE OF HARDNESS. + 7-3 7-0 7-0 7-0 6-5 6-2 6-3 6.0 6-0 6-0 5-2 As the property of hardness is of great value in the case of precious stones, those kinds which are scratched by quartz, and which, consequently, are below 7 degrees of hardness, are ranked as half- hard, or " demi-dures." Stones scratched by a knife are below 5 degrees. There are two remarks as to degrees of hardness which it is proper to introduce in this place. Firstly, the degree of hardness of a crystal or a cut stone varies, generally, however, within narrow limits, on different faces and in different directions. Secondly, the usually accepted scale of hardness is one having very different values for the different intervals. Unlike the degrees of the thermometer where the interval between one degree and the next above it or below it has the same value, whatever part of the scale be chosen for comparison, the degrees of hardness on Mohs' scale show extraordinary divergences. Specific Gravity. The most generally applicable of all modes of discriminating precious stones from one another is to ascertain their specific gravity that is, the relative weights of equal bulks the weight of a bulk or volume of distilled water (commonly taken at 60 F. or I5'6 C.) being employed as the unit with which all the others are compared. There are three modes of ascertaining the specific gravity of a stone : (i) By placing it in heavy liquids of known specific gravity, and noting the position which it takes up. (2) By weighing it in air, and then in water (or other liquid), and I 4 PRECIOUS STONES. thus learning the weight of water which the stone displaces that is, the weight of an equal bulk of water. (3) By measuring or weighing, directly or by difference, the water which the stone dis- places when immersed in water in a small vessel of known capacity. We will now briefly describe these three methods. i. Several different liquids have been used for the purpose of ascertaining the density of minerals, and even for separating species having different densities from one another. One of these liquids, which has done good service in its day, is a saturated solution of potassio-mercuric iodide. This may be prepared so as to have a density of 3*18 at 15 C. It is a yellow liquid called after its dis- coverer Sonstadt's Solution. Unfortunately this liquid is very poisonous and rapidly destroys, by amalgamating the metal, any brass apparatus with which it may come into contact. Two sub- stitutes for this liquid are now in use. One is an aqueous solution, which may be diluted at will with water, of the compound known as cadmium boro-tungstate. The crystals of this salt, to which the formula Cdj W a B 2 O 31 , 2 H 2 O + i6aq. has been assigned, when fused, over a water-bath in their own water of crystallization, yield a liquid which at 75 C. has the specific gravity 3 '55. At 22' C. this cadmium tungstoborate in crystals requires but i-ioth of its weight of water for solution : a very small further addition of water enables one to secure a solution which at 15 C. presents the specific gravity of 3*28. The other heavy liquid to which reference has been made, as a second substitute for Sonstadt's Solution, is methylene iodide, the formula of which is CH 2 L. This compound has the density of 3 32 at 15 C. Its density may be lowered by the addition of toluene, which, at the same tempera- ture, has the density 0*869. On the other hand, the density of methylene iodide - may be raised by saturating it with iodoform (C H I 3 ) and iodine. It is well to be content with the addition of iodoform only, for iodine makes the liquid too dark in colour for the movements of a stone put therein to be observed. It will be SPECIFIC GRAVITY. 15 seen that we have now at our disposal liquids* which present a range of density sufficiently wide to permit of the identification of minerals having densities up to about 3*4: and this result can be achieved without having recourse to those liquids which need to be warmed above 15 C. in order to maintain them in a liquid con- dition. It ought to be mentioned that the methylene iodide pre- parations, owing to their volatility and to their high coefficient of expansion when heated, yield results, which, in the absence of the necessary precautions, may easily be somewhat inexact. In order to furnish a liquid which will enable one to deal with stones having a density above 3*4, the double nitrate of thallium and silver may be taken. It is better to purchase this salt ready prepared, but it may be made by melting together 150 grams of crystals of commercial thallium nitrate and 64 grams of silver nitrate along with a little water and heating the mixture with con- stant stirring until the temperature of 70 C. has been reached. It is possible thus to obtain a liquid which at 75 C has a density of 4 '8, but in practice this figure need not be reached. It must be remembered that all the dilutions of this liquid have one common property they are poisonous ; moreover they stain the skin a dark slaty purple not easy to remove. For the purpose of the collector and connoisseur in precious stones it will suffice to have at hand the follows six heavy liquids : A. Thallium and silver nitrate solution of specific gravity 4 '5 maintained at a temperature well above its fusing point. B. Thallium and silver nitrate solution of specific gravity 4'1 maintained at a temperature well above its fusing point. 0. Thallium and silver nitrate solution of specific gravity 3 '9 at 15 C. D. Thallium and silver nitrate solution of specific gravity 3*5 at 15 C. E. Cadmium boro-tungstate solution of specific gravity 3 '28 at 15 C., or methylene iodide slightly diluted with toluene. F. Cadmium boro-tungstate solution of specific gravity 2*67 : this is pre- pared by diluting E. with water until a fragment of beryl sinks and a fragment of amethyst floats therein. Or methylene iodide, diluted with toluol to the same density and with the same indicators, may bq substituted, i6 PRECIOUS STONES. The stone to be determined should be first placed in liquid A, in which all stones but red, dull-green, puce, yellow, white, and brown zircons will float. After removal from A, washing with hot water and wiping dry with a cloth, the stone (which has not sunk in A) is placed in liquid B, where, if it sinks, it may be almandine, spessartite or golden -zircon. Should it not sink, it is transferred with due precautions to C. Here, if it sinks, itmay be ruby, sapphire or one of the other varieties of corundum, or possibly a green zircon. If, however, the stone floats in C it may belong to one of the much larger groups, with which we will now endeavour to deal in a tabular form : In solution D Diamond } Topaz Spinel sink In solution Jadeite Diopside Peridot Chrysolite sink sink, while float Chrysoberyl Alexandrite Pyrope Demantoid > In solution F Beryl Emerald Turquoise Phenakite Jade Tourmaline Spodumene Much may be learnt by the behaviour of a stone in the liquid employed. It may sink or rise slowly when its specific gravity is near that of the liquid, or it may remain, as it were, suspended in the midst, in cases where its density is the same as that of the liquid. Before using any of the solutions their specific gravity should be carefully determined , they must be preserved from /Opal Moonstone ' Lapis-lazuli lolite Amethyst SPECIFIC GRAVITY. 17 dust, evaporation, etc., in suitable stoppered ancf capped bottles or short wide tubes in the case of A and B. A number of weighted glass bulbs or a series of small mineral fragments of ascertained specific gravity are very useful as " indicators." It is a good plan to keep one or more of these indicators in each liquid to be employed. To avoid doubt and confusion these indicators, whether bulbs or mineral fragments, should present so characteristic a form or colour or marking that their identity and value can be recognized at once. It is worth while adding the remark that liquids A, B, and C, are re- quired much less frequently than the less dense liquids, and that when the position of a doubtful stone has been once fixed by the density test so as to prove that it belongs to a particular group, then it may be necessary to call in the aid of the dichroiscope and of the scale of hardness in order to learn to what species in that group the stone really belongs. 2. By weighing a stone in air and then in some liquid of known density, the weight of the bulk of the latter displaced by the stone is ascertained. If, for example, a sapphire weighing 4 grains in air weighs but 3 grains in water, it has evidently displaced i grain of water, becoming lighter by that amount. So the number 4 represents the specific gravity of sapphire, showing, as it does, the number of times that the weight of any bulk of that stone contains the weight of an equal bulk of water. An example of an actual experiment of this kind will serve to illustrate this, the ordinary method of taking specific gravities, better than any further ex- planation of the principle involved. A yellow sapphire weighed in air . . 12.896 grams. water . 9.677 Difference, that is, weight of water displaced 3-2IQ The proportion will be : VVt. water displaced. Sp. gr. water. Wt. sapphire. Sp. gr. sapphire. 3-219 : 1 12-896 x .= 4-006 8445. i8 PRECIOUS STONES. There are several corrections which are needed before an exact result can be reached. They are these : Firstly, the stone and the water must be compared at the same temperature, usually that of 60 F. or 15 '6 C. This is the most important correction and the only one usually applied ; it is well to avoid the necessity of introducing it, by conducting the experiment at the standard temperature. The second correction originates in the fact that the stone is weighed in air, and consequently is buoyed up to some extent by that fluid, appearing lighter than it would be if weighed in vacno. The third correction depends upon the material of the weights. These, if of brass, displace from one-half to one-third of the amount of air displaced by the stone in the other pan of the balance, and consequently involve another error. The several corrections we have named may be learned with sufficient accuracy by the following methods : The correction for temperature may be applied by multiplying the difference between the weight in air and the weight in water, not by unity, but by the actual specific gravity of water at the observed temperature,* then proceed with the calculation as before. The correction on account of the air and the brass weights is given by the formula : y = w -0012 (a - ' 12 ) where w is the observed weight in air of a given substance ; d its approximate specific gravity ; * 0012 the mean density of atmo- spheric air ; * 12 the reciprocal of the specific gravity of brass, and * If the specific gravity of water at 4 C. be taken as 1, gravities at higher and lower temperatures will be : then the specific 99987 7 99993 14 99930 1 99993 8 99989 15 99916 -2 99997 9 99982 16 99900 3 99999 10 99975 17 99884 4 1-00000 11 99966 18 99865 5 99999 12 99955 19 99846 6 99997 13 99943 20 99826 SPECIFIC GRAVITY. IQ y the weight by which the substance when weighed with brass weights will appear too light. The true weight, W, in vacuo will then be : W = w + y Now, with the true weight, W, in vacuo, the specific gravity may be calculated according to the equation previously given. To furnish a notion of the value of this correction, it may be stated that a fragment of rock crystal weighing 10 grams will become 10*0031 grams, a grain of 3 parts in 10,000. When the specific gravity of a small gem is to be taken, an assay balance of great accuracy may be advantageously employed. f In this case the full advantage of the delicacy of the instrument cannot be secured if water, which has a high surface-tension, be the liquid in which the stone is weighed, the friction between it and the stone and immersed pan being too great. Alcohol considerably diluted with water answers well. A fair quantity is prepared and preserved in a well -stoppered and capped bottle. Its specific gravity is best ascertained by means of Dr. Sprengel's tube. In the follow- ing example of an experiment a dilute alcohol of sp. gr. '8488 at 15 C., and containing about 80 per cent, by weight of absolute alcohol, was used : Specific gravity of brilliant-cut specimen of phenakite. Weight in air . . . i . 1294 gram. ,; alcohol at 15 C. 0.8064 Alcohol displaced 0.3230 ,, The equation will be : 1 -12Q4 x -8488 . 3 * 3 = 2-9676 = sp. gr. of phenakite. The one objection to this use of diluted alcohol consists in the tendency which it has to change its density by loss of alcohol ; on this account pure toluene, a liquid hydrocarbon, having a density of '869 at i5C., affords a convenient substitute for spirit. t M. Jolly's spring balance, as modified by Mr. C. F. Cross, is another useful form of instrument for this purpose. 8445. B 2 20 PRECIOUS STONES. 3. The third method of taking specific gravities does not admit of great exactness. A small wide-mouth bottle or beaker, with a ground rim and ground glass cover, both truly plane, is filled with water, the cover placed in position, avoiding air-bubbles and wiping off any water outside the vessel, and then weighing it and its contents. Let this weight be x. Now introduce the gem and replace the lid as before ; let the present weight be y, and that of the gem in air w ; then approximately, = sp. gr. w + x y In employing this method the vessel used should be no larger than will contain the specimen. Specific gravities may be ascertained by means of contrivances dependent upon the measurement of the liquid the objects dis- place from a vessel of known capacity or carefully graduated. The space at our disposal will not allow of any further details on this subject. But a caution as to the necessity in all specific gravity experiments of getting rid of air-bubbles may not be out of place. To attain this end boiled water should be used, and if mechanical contrivances fail (a feather or sable pencil) then the liquid and stone should be placed under the receiver of an air-pump and the air exhausted. Details concerning the specific gravity of each kind of precious stone will be found in chapter vii. The following table gives a fair number of average densities arranged in regular sequence : FOUR AND ABOVE, Haematite Pyrites . . Zircon Almandine Sapphire . THREE AND ABOVE. TWO AND ABOVE. 5-3 Green garnet . 3-85 Phenakite . . 2-98 5-2 Chry so beryl . . 3-76 Turquoise . 2-75 4-6 Pyrope . . 375 Labradorite . . 2-72 4-2 Hessonite . 3-66 Beryl . . 2-70 4-0 Spinel . . 3-65 Amethyst . 2-66 Topaz . . 3-55 Rock crystal . . 2-65 Diamond . 3-52 lolite . . 2-63 Peridot . . 3-38 Moonstone . . 2-58 Spodumene . . 3-20 Opal . 2-20 Tourmaline . .. 3-10 LUSTRE. 21 The following brief notes as to the physical or* mechanical pro- perties of minerals not already discussed or described must suffice . Form. The forms of crystals are all referable to one or other of these six crystallographic systems, (i) The cubic or monometric; (2) the pyramidal, dimetric, or tetragonal ; (3) the rhombohedral or hexagonal ; (4) the prismatic, trimetric, or orthorhombic ; (5) the oblique or monoclinic ; (6) the triclinic or anorthic. Structure. The mode of mechanical aggregation or intimate texture of minerals may often be learnt by disruption of the mass, or by splitting or cleaving it. Structure is often crystalline, laminar, fibrous, or columnar. Fractured, not cleaved, surfaces are less instructive they may be conchoidal, uneven, splintery, or hackly. Transparency. For want of a more comprehensive term, the various degrees of resistance to the transmission of light through minerals are included under this title. The degrees are five : Transparent when objects can be seen distinctly. Semi-transparent when objects can be seen dimly. Translucent when light, not objects, can be seen. Subtranslucent when light is transmitted through thin splinters. Opaque when light is not transmitted. Lustre. This character, although it needs some practice to discern it accurately, is of importance as an element not merely of the, beauty but also in the discrimination of precious stones. The terms employed to designate its various qualities are these : Metallic, as on pyrites. Adamantine, ,, diamond. Resinous, ,, garnet. Vitreous, ,, emerald. Waxy, turquoise. Pearly, moonstone. Silky, crocidolite. 22 PRECIOUSVSrONES. Metallic and adamantine lustres are connected with high re- fractive indices. The colours of precious stones are discussed in chapter iv. In the five tables which follow have been arranged certain physical data of precious stones so that they may be discriminated from one another by comparing their specific gravities, their behaviour when examined with the dichroiscope, and their hardness. DISCRIMINATION OF PRECIOUS STONES. WHITE STONES. NAME. I DENSITY. j PLEOCHROISM. HARDNESS. Opal , . - / . . . 2-20 N( me 6 Moonstone . . . 2-58 6 Rock-crvstal . . . 2-65 7 Beryl ". . . . .. 2-69 7| Phenakite . . 2-98 : 7| Diamond . . . . 3-53 10 Topaz . . . ... 3-57 8 Sapphire . . . '' * 4-00 9 Zircon . . . ... 4-75 74 RED AND PINK STONES. NAME. DENSITY. PLEOCHROISM. HARDNESS. Tourmaline . . . -i. 3-05 Strong 71 Kunzite . . . ' . 3-18 ,, 6f Topaz . . . : . 3-53 ,, 8 Spinel . . ... ;}-58 None 8 Pyrope . ... 3-75 ,, 74 Ruby . . . 4-00 Strong 9 Almandine . 4-15 None 71 Zircon . ... 4-70 Weak H DISCRIMINATION OF PRECIOUS STONES. 23 ORANGE AND YELLOW STONES. ^ NAME. DENSITY. PLEOCHROISM. HARDNESS. Cairngorm . . . 2-66 Weak 7 Beryl . ' . . . . ' .. 2-69 Strong 71 Tourmaline . . 3-11 ,, 74 Spodumene . . . . 3:20 Weak 6? Diamond . . 3-53 None 10 Topaz . . . ' 3-53 Strong 8 Hessonite . . . 3-66 None 7 Chrysoberyl . . . .*'.' 375 Strong 84 Sapphire . , . . . ~ 4-00 ,, 9 Spessartite . . . . 4-15 None 7i Golden Zircon . . .' 4-40 Weak 7 Yellow Zircon . . : . 4-67 M 7i GREEN STONES. NAME. DENSITY. PLEOCHROISM. HARDNESS. Emerald 2-70 Strong 71 Aquamarine . . . 2-70 71 Jade 3-00 Distinct 6 Tourmaline . 3-11 Strong 7| Hiddenite 3-15 it 6| Jadeite 3-32 Distinct 7 Diopside 3-34 Weak 6 Peridot .... 3-40 Strong 6J Epidote .... 3-44 > 6i Spinel . 3-58 None 8 Alexandrite . 3-65 Strong 8* Demantoid . 3-85 None 6 Sapphire 4-00 Strong 9 Zircon .... 4-05 Weak u PRECIOUS STONES. BLUE AND VIOLET STONES. NAME. DENSITY. PLEOCHROISM. HARDNESS. Lapis-lazuli . lolite : 2'40 2-63 None Strong u Amethyst . . . Beryl . 2-66 2-69 Weak Strong 7 71 Turquoise Tourmaline . 2'75 3-10 None Strong 6 Topaz .... 3-55 ,, 8 Spinel . . 3'65 None 8 Sapphire , . 4-00 Strong 9 Zircon .... 4-65 Weak 74 On pages 6 to 20 there have been described easy methods of ascertaining the density, pleochroism and hardness of precious stones. The index of refraction may also now be determined, thanks to Mr. G. F. Herbert Smith's improved Keiractometer, which is available for all gems of which the index of refraction lies between T45 and T74. The instrument, as made by Mr. J. H. Steward, is accompanied by instructions how to use it and how to interpret the observations made ; also by a table of indices corresponding to each division of the graduated scale. The Refractometer is standardized for use with the yellow light of a sodium flame, but white light may be employed if approximations suffice. Place a small drop of a highly refractive liquid a bromonaphthaline, for example upon the plane surface of the hemispherical lens. Then press a flat face of the gem into the liquid, so that the latter has a uniform thickness. Light being admitted through the ground glass " window " of the instrument, and the scale focussed, it will be seen that different parts of the scale are differently illuminated ; the edges or boundaries of these regions where they coincide with the scale-divisions are to be noted. With glass and other singly- refracting substances, the edges due to these solids are single, with doubly- refracting minerals double. It is essential that the edge arising from the refractive liquid itself should be beyond the edge due to the stone. So when spodumene, peridot and spinel, with indices ranging between 1 *65 and 1 '73, have to be tested, methylene iodide, with a refractive index of 1 "74, must be substi- tuted for bromonaphthaline, the refractive index of which is only 1 '658. CHAPTER III. CUTTING AND FASHIONING PRECIOUS STONES. VERY few precious stones, as we receive them from the hands of nature, present the beautiful qualities for which we look in these concentrated treasures of the earth. Often they are waterworn pebbles, roughened by attrition and blows during years or even centuries of wanderings in the beds of streams and rivers. If we find them intact in their rocky homes, they are oft-times obscured with flaws and intruding matters which mar their beauty. If trans- parent and without speck or fracture, yet the natural forms in which crystallised gem-stones occur are but rarely adapted for direct employment in objects of jewellery. In shape or size they are awkward for such use, while many of those marvellous optical qualities which distinguish them from the crowd of commoner materials are brought into prominence only by the artificial processes of cutting and polishing. These processes convert rough crystals into shapely gems, having fine qualities of surface lustre and interior colour, and, withal, much less liable to fracture than the original stone. Now and then a perfect natural octohedron of flawless diamond or rosy spinel may be set in a ring or jewel ; but such instances are exceptional, and gem-stones, in order that all their elements of beauty may be developed to the uttermost, must be cut and polished according to rule. All the different forms into which precious stones are cut may 26 PRECIOUS STONES. be arranged into the two groups (i) those having plane surfaces ; (2) those having curved surfaces : but, under special circumstances, facets or plane surfaces are occasionally associated with curved surfaces in the same specimen. The further subdivision of the two groups of forms may be tabulated thus : Group 1. Plane surfaces Group 2. Curved surfaces Brilliant-cut. Step or trap-cut. Table-cut. Rose-cut. Single cabochon. Doable cabochon. Hollowed cabochon. Tallow top. A few words descriptive of each of these modes of cutting stones may now be given. The old brilliant-cut, though susceptible of many small modifi- cations as to the size of the facets, their mutual proportions and inclinations, and even their number, requires, when perfect, 58 facets thus arranged : 1 Table 8 Star facets 4 Templets or bezels 4 Quoins or lozenges 8 Cross or skew facets 1 8 Skill facets* 33 The " Crown," or upper part of the brilliant. Fig. 12. * The cross and skill facets are sometimes called half-facets ; the former are known as cldtures by the French lapidaries. BRILLIANT-CUT. 2 7 1 Culet or collet 4 Pavilion facets 4 Quoins 8 Cross facets 8 Skill facets = The " Pavi- lion," or base, or under part of the brilliant. Fig. 13. There are thus 58 facets in a brilliant, while the " girdle " or edge bounding the widest part of the stone divides the crown from the base, and is concealed, in part at least, by the mounting or setting. This girdle must not be very thin (it is liable to be so in what are called " spread," that is, shallow, stones), for then it may become chipped and break away during mounting. If it be thick, on the other hand, the brilliancy of the stone is lessened, and its material wasted by the concealment of a good deal of it in the mount. This form of cutting is reserved particularly for the diamond so much so, that the word "brilliant" used alone signifies a diamond cut after this fashion. Of late years the girdle of brilliants has been made to approach a circular outline ; the templets and quoins are nearly of the same size, and eight star facets are cut round the culet, thus making a stone of 66 facets. Certain rules have been laid down for the relative proportions, not only of the several classes of facets in a brilliant-cut diamond, but also for the thickness of the finished stone in each and all its diameters. Thus i-3rd of the total thickness should be occupied by the crown or upper portion above the girdle, 2-3rds being below. The table should be 4~9ths of the breadth of the stone, and the PRECIOUS STONES. culet i -6th to i-5th of the table ; bu-t according to some modern experts, both these facets, but chiefly the former, may be reduced with advantage below these proportions. Two of the most famous diamonds of the world show large departures from the typical proportions of a brilliant : The Koh-i-nur in its present form is far too broad for its depth or thickness ; the Regent is a good deal too thick for its breadth. But the same rule of proportion, although it may hold good for such diamonds as admit of being subjected to it without extravagant loss of weight, must be modified with stones of other species, and especially with coloured stones. With colour- less topazes, sapphires, etc., the surfaces and inclinations of the facets must be modified to suit the refactive indices and other optical constants of these minerals ; with coloured stones, if pale (certain alexandrites for example), greater depth must be secured ; if dark in hue, then greater " spread " and less depth (deep red garnets furnish instances). The style of cutting known as the step-cut or trap-cut is adopted for the emerald and many coloured stones. It is subject to rules of proportion far less strict than those devised for the cutting of the diamond in the brilliant form. Each species of stone needs special study, that the typical step-cut may be so modified as to bring out the full beauty of the gem. The fault most common with step-cut stones is the too great breadth of the table, for the in- ternal reflections from the lower facets are best seen through the sloping bezels of the crown, not through the flat sur- face of the table. In the step-cut (fig. 14) we have then a table, two or more sloping step facets, and then the girdle, while the lower part of the stone (fig. 15) is cut into three or more sets or zones of Fig. 15. Fig. 14. CABOCHON. 29 diminishing steps, with an oblong square or Hexagonal or octagonal culet as termination. Some trap-cut stones are brilliant cut below the girdle, or vice versa. The table-cut needs little description : it has a very largely developed table with bevelled edge, or a border of small facets. It is employed ioi covering fine gold-work and miniatures ; in the sixteenth century and later it was used in Europe for much diamond-work. The rose-cut (fig. 16) shares with the table-cut a much greater antiquity than the brilliant-cut. It may be compared with the latter by supposing the table to be replaced by six triangular or star facets, and the crown to be represented by eighteen tri- angular cross and skill facets which together con- stitute what the French call la dentelle. The F - 1( , base is either flat or a duplicate of the upper part. The other forms given to faceted stones are not of sufficient importance to need description ; the star-cut and the pendeloque may just be named as patterns sometimes followed in the cutting of diamonds. Translucent and opaque stones are commonly cut en cabochon (fig. 17); the opal and the turquoise are characteristic examples. The moonstone, avanturine, cat's-eye, and star sapphire, too, would not show their peculiar properties were the confusing reflected lights from facets to be mingled with the white sheen, the brilliant spangles, the silver thread, or the six-rayed star which these stones respectively present when properly fashioned. The one transparent stone which is frequently cabochon-cut is the garnet, which is then called a carbuncle. A variety of cabochon used for this gem is 30 PRECIOUS STONES. somewhat hollowed behind (fig. 19), to receive a piece of foil as well as to lessen the depth of colour in very dark stones. Our figures represent the simple cabochon (fig. 17), the double cabochon (fig. 18), the hollowed (tvidt) cabochon (fig. 19), and the flattened form much used for opals, and called tallow-topped (fig. 20). The double cabochon is usually cut with the base of lesser ' Fi^ 19 curvature than the crown ; but with many stones a more brilliant play of coloured light within the stone may be secured by reversing these proportions. Although the cabochon form is almost essential to many precious Fig. 20. stones, and is useful to hide the poverty and flaws of others, and also is convenient in the case of stones to be used in the decoration of vases and other objects to be handled, yet it ought not to be allowed to displace the various faceted forms. Doubtless there is a quiet beauty and richness in a good cabochon ruby, sapphire, emerald, or jargoon, but we lose some of the most striking characteristics of these gems when we so cut them as not to admit of the display of their dichroism, and their dispersive and reflective powers. The narrow view that all faceted stones are vulgar is based on caprice and ignorance ; it is the mere un- intelligent whim of a clique of artists and amateur writers on art. For the faceting of the great majority of transparent stones is an operation necessary for the development of those optical qualities upon which the beauty of precious stones mainly depends. It should be performed in strict accordance with certain rules of proportion, which may be deduced from the optical constants of each species of stone. Information as to the mechanical processes and the materials employed in the cutting and polishing of precious stones may be found in the works of Jannettaz and Dieulafait. Horizontal wheels of steel, gun-metal, copper, lead, pewter, tin and wood CARBORUNDUM. 31 charged with various grinding and polishing pofvders, are employed for different stones, and in different stages of the operations. The wheel, or disc, or lap as it is called, is usually horizontal and is made to revolve with great rapidity. The grinding or polishing powder, mixed, according to its nature, with olive oil or water, becomes partially embedded, in the lap. This powder, in the case of diamonds, must be of diamond itself, generally in the form of boart, a dark and rather porous variety of the mineral. The com- paratively new and artificial compound of silicon and carbon known as carborundum is now largely used in the case cf the harder stones, but emery, garnet-powder, tripolite, rotten stone, jeweller's rouge, pumice, putty-powder, and bole are in constant requisition for the grinding and polishing of stones less hard than the diamond. The whole subject of this mechanical treatment of stones, including splitting, dividing and shaping operations, is one which cannot be discussed here, involving as it does a large number of minute tech- nical details of no interest from an artistic standpoint. CHAPTER IV. ARTISTIC EMPLOYMENT OF PRECIOUS STONES. SOME acquaintance with the less obvious characters and qualities of precious stones, and especially with the distinctive properties of those kinds which remain practically unrecognised and unap- preciated, may serve more than one good purpose. Not only may the jeweller's art receive new impetus and suggestion, but the buyers and connoisseurs of bijouterie may learn to appreciate more highly well- conceived design, new combinations, and ex- quisite workmanship. Most admirable and pleasant colour-com- binations may be attained by the aid of materials which in many instances are now by no means costly. Curious and delicate hues of luminous and refined quality, preserved in enduring substance, may be arranged and grouped in forms of endless beauty and variety. Neither silks, nor paints, nor even enamels can ever equal the colours of precious stones in durability, or in brilliancy and pulsating variety of hue. And it cannot be doubted that when knowledge of the true nature of any art material (such as precious stones) becomes more intimate, exact, and diffused, a more intelligent and lively interest will be created in examples of good work wrought in the substance in question. Every connoisseur or collector of artistic objects must have shared in experiences of this kind. He may have been once quite dead to the peculiar merits of certain works, say in bronze, not even glancing at any specimens falling in his way. Then some casual circumstance, perhaps an exciting contest for a fine piece of work at a sale APPRECIATION OF PRECIOUS STONES. 33 between two enthusiastic collectors, or perhaps 'the gift of a choice specimen, may have drawn attention, not perhaps to the merits of such specimens, but at least to the esteem in which they may be held. Curiosity it may be an intelligent curiosity is excited. Investigation, more or less searching, follows. The hardness of the metal, its provenance, its designer, its age, the mode of manu- facture, whether by casting or hammering ; the manner of decora- tion, whether by chasing, engraving, or inlaying ; the colour and texture of the surface, the presence or absence of patina ; and not a few other points of interest, constitute the materials of complex study. Study provokes observation, and observation study, so that before long the neglected group of artistic bronzes exerts a kind of fascination upon the new votary. If his knowledge be superficial and inaccurate, or if he be merely an ameteur or col- lector just because it is a fashionable pursuit to gather together or to admire certain classes of artistic objects, well then, he does not really know what and why he admires. Forgeries delight him just as much as genuine works, so long as he is not sure that they are forgeries ; but he has not sufficient patience for the mastery of, or sufficient insight into, the characteristics of true productions to discriminate them from those that are false. It often happens thus with the amateur of precious stones. He knows nothing of the optical elements, say of surface lustre, and the pleochroism which go to make up the tout ensemble of any particular gem, and is quite satisfied with a well-cut bit of paste, or a cleverly contrived doublet. No doubt, in some cases, even an educated keenness of vision does not suffice to distinguish the true stone from the false, although the durability of the genuine specimens will ultimately prove their superiority. But it is not difficult to learn to appreciate the peculiar and essential characters of the majority of the species of precious stones. The few simple pieces of apparatus and the appliances described in the second chapter, will serve to supplement and correct the deductions of a 8445. C 34 PRECIOUS STONES. trained eye and touch. And with a spectroscope, a polariscope, in addition to a good hand magnifier or pocket lens, such an array of evidence may be marshalled that there can remain but few cases in which the identity of a stone shall continue doubtful. But for the purpose of the artistic employment or appreciation of precious stones such a table as that given on pages 5 and 6 will prove more useful than any recondite method of inquiry. Some of the uses of that tabular arrangement of conspicuous optical qualities may be gathered from the following examples. Referring to the shape of stones we note that their boundaries are either plane or curved. Now if we have to use, in any piece of personal ornament, stones having curved surfaces, it will not answer in general to associate with them other curved surfaces, like those of the en cabochon moonstone ; and especially is this the case where the size of the stone, as well as the character of the curved surface, is nearly identical ; but a happier result will be attained by combining a step-cut stone with one having a curved surface. Again, citing an example from the series of adjectives expressing qualities of sur- face, it will be found that gems having an adamantine lustre assort better with those which present the less brilliant surface known as waxy, than they do with those which show a nearer approach to the adamantine surface, and which are called resinous. The diamond and the jargoon do not improve or bring out each other's qualities, for they have too many points in common ;- but the diamond accords well with the pearl, and the jargoon with the turquoise, that is, the adamantine with the pearly, and the re- sinous with the waxy. Looking, now, into the substance of stones, rather than on their surface, their relations to the transmission of colourless light furnish many illustrations of wise and unwise, or effective and defective combinations. For example, chatoyant stones, like cat's-eyes, do not associate well with translucent stones, like the chrysoprase and the chalcedony the translucency of the latter confuses, because it resembles too closely, ] the chatoyancy ARTISTIC ASSOCIATIONS. 35 f of the former. But transparent stones accord well with all those which interrupt the passage of light by such internal reflections. The diamond, on this account, combines admirably with the cat's- eye and the pearl, but it affords too strong a contrast, especially when of large size, with the turquoise, to associate pleasantly with this nearly opaque stone. From amongst the qualities pertaining tc the colour of stones, examples of the utility of the table may be cited. When a stone has much " fire " in it- -that is, when its refractive and dispersive actions upon light are high and it shows prismatic hues, then it looks best if associated with gems in which this property is less developed. Again, monochroic stones, which in all directions transmit beams of the same colour, should be associated with pleochroic stones, which exhibit two or more hues, while the latter should not be mixed together. We are led from the study of these examples of associations of gem-stones to inquire into the principles which underlie artistic combinations. Probably we are satisfied with arrangements of precious stones in which the leading motif is either identity, or seriation, or contrast. When stones match, when they are gra- duated, or when they offer a distinct but not startling contrast, the resulting effect is at least capable of being made satisfactory. When we speak of identity, seriation, and contrast, as expressing the elements of decorative association in the mounting of precious stones, we use words into which we are compelled to import special meanings. By identity, we mean that very close resemblance which selected specimens of choice stones of the same kind will exhibit ; seriation expresses the orderly sequence of tones or colours with the presence of a pervading and dominant element ; contrast implies an effect of change rather than of passage , and may include contrast of tone and of lustre as well as contrast of colour. Instead of further discussing the question of the artistic employment of precious stones in precise accordance with the three principles of association before laid down, a more useful and 8445. C 2 36 PRECIOUS STONES. generally available plan will be to follow a classification according to colour. For as the ornamental or artistic employment of precious stones conveys primarily, if not wholly and ultimately, an appeal to the eye, it is clear that such optical properties as can be comprised in the terms lustre, light, and especially colour, should be our first consideration. After all, as, on the whole, the prominent feature of precious stones is their colour, so the easiest way of considering their colour is to adopt the order of succession of the colours in the ordinary rainbow or prismatic spectrum, beginning with the white light, which contains them all, and origi- nates them all. White Stones. The diamond naturally takes the first position if we consider its hardness, its remarkable composition, and its strong refraction and dispersion of light. Its properties, so far as they appeal to the eye, differ much from those belonging to the majority of other stones, and it forms, partly in consequence of this peculiarity, as good a border or setting to other gems as a gold frame generally does to a picture. Of course much depends upon the quality of the diamond, and much upon the shape which is given to it by the lapidary. The flat plates of lasque diamonds, and, in less degree, the step-cut stones with broad tables, exhibit the unique and splendid lustre which is peculiar to the polished surface of this stone ; these forms also permit the transparency and the total internal reflection of light to be well seen. Even the form of the diamond crystal, the regular octohedron, when its sur- faces are really planes, well exhibits the transparency and reflection of the stone. Next to the diamond we may place the colourless zircon or jargoon, then the phenakite, then the white sapphire, the white topaz, and the white beryl. Rock crystal will come below these in point of beauty and brilliancy. The colourless zircon sometimes approaches near in prismatic brilliancy to a dia- mond ; so, at night especially, does the rare and curious mineral phenakite. There is, however, always a sort of difficulty in finding WHITE STONES. 37 an appropriate use for colourless, yet lustrous', stones in any article of jewellery intended for personal adornment. The more lustrous and prismatic they are the more they resemble the diamond, in fact the less available are they for the usual purpose to which gems are put. Still, there are peculiar qualities in these stones which need not be lost to artistic employment, if the white stones in question be judiciously associated with materials which pre- vent their being mistaken for diamonds. A white diamond should rarely or never be bordered by green tourmalines, but these stones would form an agreeable combination with a white zircon, a phen- akite, or a white topaz. In the white sapphire there is often a faint suspicion of milkiness, and in the white beryl a cool green- ish tint, which prevent these stones from resembling the diamond so closely as to be taken for imitations of that gem. But many of these colourless stones, notably the topaz and rock crystal, in all probability are most appropriately used when set as bosses in vessels and other large pieces of metal work, or employed in the form of plaques for engraving or etching. It is scarcely neces- sary to justify such uses of these minerals, and this is not the place to enter upon the question, particularly as it is only by a rather wide use of the term precious that I am able to include these materials, and some others which I shall have to discuss presently, amongst precious stones. Of two other white materials em- ployed in jewellery, the moonstone and the pearl, a few words may be introduced here. The moonstone forms an excellent substi- tute in many combinations for the pearl, but it does not associate so well as the latter with the diamond. With deep-coloured amethysts, spinels, and tourmalines, few colourless gems look more refined than the moonstone. But these stones, which fetch a shilling or so apiece only, should always be accurately recut and highly repolished before being used. Their forms are too irre- gular and their surfaces too imperfect, as imported from Ceylon, to show off their moonlight sheen with half its intensity, unless 38 PRECIOUS STONES. they are passed again under a careful lapidary's hands. The improvement thus effected is marvellous. The value of the pearl, whether its " orient " be luminous with prismatic hues, or whether it be a warm soft white merely, is too well known to be more than named in this connection. But we may be permitted to say one word in deprecation of the extravagant expenditure of time, of ingenuity, and of costly materials, which the attempt to convert large irregular pearls into structures resembling figures has so often caused. The result is nearly always most unhappy. Red Stones. The ruby may fitly be considered before other coloured stones. It, with the sapphire, and all the transparent varieties of corundum, ranks next to the diamond in hardness. It is, moreover, a stone of great beauty. Probably the experts in jewels are right in assigning the highest value to those rubies which possess a " pigeon's blood " colour this is the orthodox hue. But the paler colours, and those which verge upon pink and crimson, and even violet, are capable of being so treated by means of association with white and black enamel or with dark stones, like olive-green tourmalines, as to lend themselves to the production of very beautiful decorative effects. The great mistake commonly made in the treatment of the paler rubies lies in the attempt to treat them in the same way as the deeper coloured stones. It is difficult to describe the peculiar colour quality of the ruby in words. In fact, our nomenclature of colours is neither ample nor accurate. Our appreciation of delicate differences between colours is growing, but the language by which we endeavour to describe the hues which we have learned to appreciate is either stationary, or else receives additions from time to time of unsatis- factory words, derived from the caprices of French fashions. The time has really arrived when a standard series of hues of all sorts should be constructed and appropriately named ; but, in the case of the ruby, the question of pleochroism comes in, and renders RED STONES. 39 the difficulty of describing the colour quality ^f this stone greater. There is also some prismatic " fire " in the stone, and much in- ternal reflection of light, while its surface lustre lies between re- sinous and vitreous. These four properties give to the red of the ruby a peculiar richness, which the two other species of pre- cious stones the spinel and the garnet which come nearest to it in colour, do not equally possess. The two reds which make up the colour transmitted by the ruby do not differ much, but yet they help to impart, to a properly cut stone, a delicate variation of hue which is not present in any other red stone, nor in any imitative substance. The dichroiscope, consequently, never fails to discri- minate between a ruby on the one hand and a spinel or a garnet on the other. The two latter stones are, of course, softer than the ruby, and the former is always lighter, that is, of less specific gravity. For the ruby and the whole of the corundum family of stones have the specific gravity of 4, and a hardness which is nearly, and in some cases quite, 9 on the mineralogical scale. One of the happiest uses of the ruby is in the form of an inlay, in certain gold vessels of Indian origin. The external surface of these vessels is covered with a system of interlacing ridges and furrows. The rubies, generally small, oval, and cut en cabochon, are set along the furrows. Thus they are much protected from the chance of dislodgment, while the effect they produce, of a rich deep crimson groundwork over which a gold netting has been thrown, is in perfect harmony with the materials and their work- manship. For, naturally, the metal gold, when pure, or nearly pure, throws a ruddy tint when light is reflected from surface to surface ; witness the interior of gilt vessels. The same thing occurs in the golden furrows of which we have spoken, where the rubies seem to rest in a golden sheen, of a hue in which the yellow, and orange, and red elements, now one and now another, appear to prevail. The gold should not be burnished where much con- trast between the metallic surfaces and the rubies is desired, but 40 PRECIOUS STONES. the stones themselves should be as brightly polished as possible, in order not only to develop the full beauty and variety of their colour, but also the very considerable surface lustre which the ruby possesses. There is another kind of Indian jeweller's work to which most of the remarks I have just made apply. A per- forated plate or disc of delicate arabesque or radiated work is found decorated with ruby beads, round or oval, attached to the circumference of the ornament, or else introduced into its midst in concentric circles. Here dull dead or " matt " gold is par- ticularly appropriate, as affording a pleasant contrast to the rich, smooth, and soft transparency of the rubies, which, from the manner of their mounting, may be looked through. The refine- ment of the slender gold-work, which, in this class of jewellery, approaches the delicacy of filigree, sets off by its minuteness of detail the simpler and bolder forms of the plain, smooth, rounded stones, which give it colour and warmth. We must dwell for a moment or two upon another Eastern method of dealing with the ruby the use of this stone as an inlay or onlay that is, an in- crustation upon jade, both white and green. It is not so much here a beautiful contrast of colour that is attained, although the greenish grey, or olive green of the jade, enhances the redness of the ruby ; but it is a contrast of textures, a contrast of surfaces, a contrast of translucencies. You see but a little way into the jade, though it is illuminated by a soft diffused light ; but you see through the clear deep-toned rubies, with their flashing beams of crimson. Now compare with these examples of the artistic employment of the ruby the ordinary mode in which this stone is set by English jewellers. Look at the half -hoop ruby ring, with five rubies well matched in colour, and graduated exactly in size, set close together in a regular row. You see, perhaps, a little speck of gold appear- ing here and there at each end of each stone, but nothing is made of these pieces of gold. You accept them because you know they RUBIES. 41 are necessary to hold the stones in their places, but you find neither invention nor beauty in these little bits of gold claws. In fact, they are frequently prepared by the gross, ready for the mounting of any stones, provided the shape of the latter be suit- able. Rubies, sapphires, diamonds, garnets, and emeralds are all set in the same way, not an attempt being made to adapt the amount of gold surface or its form to the specific nature of each gem. But why should not some variety and some appropriate- ness of mounting be secured for all stones ? How exquisite, and yet how strong, were the gold and enamel settings of precious stones in the cinque-cento time in Italy ! Let those patrons who desire the rather barbaric splendour of masses of rubies gratify their taste by means of jewels in which the setting is not seen at all. But surely a fine stone is worthy of a fine and originally designed setting proportioning the latter in form, in amount of work and surface, and also in colour, whether red, or green, or yellow gold, or enamel, to the shape and the hue of the stone to be set. And even small stones become quite beautiful when arranged with taste and judgment, in accordance with the con- ditions just named, and with the further condition as to collocation of individual stones in accordance with their size and shape. In pendants, and necklets, and lockets, and brooches there is room for the expression of some definite and intelligible design. The mere alternation of rubies with diamonds in rows or chequer work may, in. some instances, achieve all that is needed. But a design of more definite form may often be preferable, especially where the stones at one's disposal are of differing colours and sizes. Then one may con- struct a suitable bit of leafage or flowerage, duly conventionalised, in accordance with the nature of the available materials, into forms of more or less geometrical severity. It should be noted that moon- stones and white sapphires, in which there often lurks a faint opal- escence, accord well with rubies ; but it must be kept in mind that the size of the colourless stones which are to be associated with 42 PRECIOUS STONES. rubies in such designs as those named is a matter of much moment. It is a mistake to attempt to match the colourless and the coloured stones in respect of size, and generally of shape also. One should be smaller than the other. Large rubies with small moonstones, or small rubies with large moonstones, and similarly, square stones with round, and oblong stones with round, generally pro- duce happier effects than square with square, and oblong with oblong. Pearls accord with rubies, not only by reason of their colour relations, but also on account of their shape. In the case of rubies cut en cabochon, brilliant-cut or square step-cut diamonds will be found to yield very satisfactory combinations. A border of small brilliants or roses is a usual and a useful mode of setting off the qualities of a ruby. The colour of the pale stone is heightened by contrast with the colourlessness of the diamonds ; the richness of a rich stone is enriched, and a small stone, if surrounded by stones still smaller, becomes magnified in proportion. Next to the ruby, amongst the red stones, comes the spinel or balas ruby, an entirely different mineral species, without any pleochroism, and inferior in hardness to the true ruby. The scarlet, aurora-red, and flame-coloured spinels are the most beauti- ful , those which verge upon crimson, purple, and violet, looking dull and black at night, but showing very delicate and often rare hues by day. Red spinels accord well with small brilliants, or with larger pearls or moonstones. A fine aurora-red spinel looks well when surrounded with delicate foliage of white, orange, and black enamels. Step-cutting, similar to that employed for emeralds, accords best with the optical qualities of this stone. A biconvex lenticular form may be so adapted to this stone as to throw a good daal of soft and rich colour into a specimen which would otherwise have had little beauty to recommend it. What richness of hue the finer examples of red spinel may show is to be studied in two specimens in the Townshend collection, Victoria and Albert Museum, Nos. 1326 and 1327. GARNETS. 43 From spinels the passage to garnets is easy.'' But it is not really difficult to discriminate between the two species, even when the colours seem the same. If you have a ruby, a spinel, and a garnet together, the first will scratch the second and the second the third. The ruby will show two colours in the dichroiscope, the spinel and the garnet only one. The spinel will exhibit no black bands like those belonging to the almandine garnet, when viewed with the spectroscope. And there is a blackness, due to much absorption of light, in many of the facets of a garnet, as seen from the " table " of the stone, which will not be observed in the spinel. The garnet, unless of remarkable size or qualit}^, will hardly be deemed worthy of being mounted in the same costly way as the ruby or the red spinel, but it may be said that the same general treatment suits all these red stones. Yet there are two ways in which garnets have for long and in many places been treated, to which I may legitimately refer here. The plates of garnet so largely found in Anglo-vSaxon and Celtic jewels have remained, in the majority of cases, intact to the present day. They afford, in their breadths of soft rich colour, a pleasing contrast to the minute filigree, granulated and enamel work with which they are generally associated. The other employment of the red garnet (and it may be traced back to a far earlier date than that just cited) is as a carbuncle not neces- sarily foiled at the back. Cut en cabochon, slightly hollowed behind, and laid on a plain gold surface, the light, as of a glowing coal, quivers in the midst of a good stone. There is a lovely disc of antique gold set with five carbuncles in the Gold Ornaments room at the British Museum. In the centre is a round carbuncle boss ; then four long pointed arms, much like elongated pears, radiate from this centre, alternately with a somewhat similar series of repousse arms, beaten up from the disc of gold, and bordered with knurled wires onlaid. There is not much work in the piece ; the intrinsic value of gold and garnets is quite small, but the effect is delightful ; simple, yet rich ; solid, yet elegant. Can the same 44 PRECIOUS STONES. praise be honestly given to* modern garnet-work ? Can we feel a genuine satisfaction either in the design, the execution, or the effect of a compound big carbuncle of eight lobes, with an eight-rayed star riveted into the midst of it, the aforesaid star being of hard, poor, glittering, much alloyed gold, and containing a number of irregular fragments of defective diamonds ? The star soon gets loose, and later on the diamonds begin dropping out. But we will not pursue the history of the piece any further, and will refrain from calling attention to other obnoxious modes of using carbuncles, as in a ring with a sham gold knot on either side. Orange and Yellow Stones. Amongst orange and yellow stones we may assign the first place to the yellow zircon a stone which is sometimes found of a hue which may be aptly described as that of transparent gold. Next to this comes the yellow sapphire, after- wards the cinnamon stone, or hessonite ; and then we may place the rich sherry-coloured Brazilian topaz that kind which yields when heated the finest rose-pink stones. Then the chrysoberyl follows, and, at some distance, the yellow beryl. Few colour combinations have been attempted with these yellow stones; puce-coloured spinels associate with the yellow sapphire very happily, but there are some enamels which answer equally well. Generally a design of pale bluish-grey enamel, with minor details wrought in buff and white, develops the richness of gold-coloured stones. Here mention should be made of the very rare gem, the spessartite of Ceylon. It is of an orange-red hue, and is of most fiery brilliance, but is very seldom met with in commerce. The North American spessartites are inferior. Green Stones. There are four green stones about which something ought to be said the emerald, the tourmaline, the peridot, and the zircon. Some persons regard the green of the emerald as vulgar. It is too easy to construct a vulgar, coarse ornament out of emeralds, even if they be of fine quality. But the_ emerald, step-cut, and judiciously and quietly mounted, possesses a rich and refreshing BLUE STONES. 45 colour, just sufficiently dichroic to show passages of bluish-green with the green. Green tourmalines are much more markedly dichroic, and it is much to be regretted that, with rare exceptions, the patrons of the jeweller's art still remain ignorant, not only of the peculiarly rich and varied qualities of the colour of the tourmaline, but even of the existence of this gem-stone. With moonstones, or with grey and ivory-white enamel, long prismatic tourmalines, carefully cut, afford a delightful colour-combination peculiarly fitted for larger pieces of personal adornment, such as pendants and brooches. The so-called green garnets of the Urals, especially those which are of an olive or pistachio green, are lustrous and fiery stones, but their softness precludes their use in rings. The same objection holds good with regard to that lovely stone the peridot ; but this species occurs frequently of large size, and so is well adapted for employment in jewels not subject to much attrition. It is a dichroic stone ; it accords well with small puce, violet, or indigo spinels, also with black and white enamel ; small dark-coloured almandine garnets may sometimes be associated with peridots of fair size advantageously. The most beautiful of all green stones are those choice green zircons which show a full velvety leaf green. These always have a low density not exceeding 4- 15, and often no more than 4. They have the merit of appearing particularly bright by artificial light. White enamel, or a border of very small green zircons, enhances their beauty. The aquamarine and other pale varieties of the beryl are stones which lose nothing of their brilliancy at night. Their beauty may generally be greatly enhanced by the judicious use of creamy white enamel, with delicate arabesques of black or indigo blue. It is not often that the hue of the beryl is such as to bear the juxtaposition of other coloured stones. Blue Stones. Of these there are four that claim notice in this place sapphire, blue spinel, iolite, and lapis-lazuli. Rich yellow dead-gold settings suggest themselves for most of these materials. 46 PRECIOUS STONES. Pearls or diamonds enhance the colour of the paler sorts of sapphire, spinel, and tourmaline, but afford too striking a contrast with very richly and deeply tinted stones. A fine indicolite, step or cabochon cut, accords well with pearls or moonstones arranged as a bordering or in some conventional form ; the gold work may well receive an enrichment in the form of grey or olive green enamel. In the case of the sapphire, the twin beams of diversely-coloured light which this stone transmits the one azure blue, the other greenish straw contribute to produce the peculiarly rich quality of its velvety softness. There is a glittering coldness in all the imitations of the sapphire the timbre of their colour, to borrow a word from music, is harsh and unsatisfactory. So a recent imitation, a kind of lime- spinel made artificially, exhibits apparently the right colour, but it is flat and uninteresting. To my eye, the difference between a true sapphire and a false one is the analogue of the difference between a piece of leafage in wrought iron, and the same piece in cast iron. As to the arrangement of the sapphire in jewellery, so much depends upon its depth of colour and its precise hue, that a general rule would be fallacious. Unless it be pale, when certain green tour- malines go well with it, the sapphire may be most safely associated with pearls, diamonds, moonstones, or white topazes, the cutting and size of the stones being carefully studied. Violet and Purple Stones. The amethyst, the oriental amethyst, and the almandine garnet cannot, as a general rule, be safely associated with stones having strongly marked contrasting hues. The paler sorts of peridot may, however, be combined with deep- coloured amethysts or almandines, provided the latter be small in comparison. The use of opaque fawn-coloured, olive green, and brown enamel with violet and purple stones sometimes yields happy effects. In devising arrangements of coloured stones a mere water- colour sketch will not suffice. It is always desirable to study with the aid of the actual materials themselves stones, gold, silver, ENGRAVED GEMS. 47 enamel the sum of the effects due to lustref texture, form, size, etc., as well as the balance and distribution of colour. In any treatment, however cursory, of the topic of this chapter, the artistic employment of precious stones, some reference ought to be made to the materials used by the gem-engraver. Nearly all the minerals employed for intaglios and cameos will be found mentioned in a subsequent chapter. Most of them are varieties of silica coloured by small quantities of iron-compounds. Such are the sards, cornelians, onyxes, chalcedonies, amethysts and jaspers in which the great majority of antique gems were wrought. An intaglio well engraved in one of the more transparent or trans- lucent of these stones, say, on a rich golden of blood-red sard, shows effects of beautiful colour when viewed by transmitted light which will be sought for in vain in any faceted specimen. And then the cameos of later dates, wrought in onyx and sardonyx, present delightful contrasts of tone and hue in their different strata, utilised as these layers often were in the building up of a relief-picture. Of other minerals employed for engraving in classic times mention may be made of beryl, garnet and plasma ; the harder and rarer stones were, however, little used until medieval and later days. It is well to remember that the jacinth, properly so-called, that is, the orange brown or brownish red zircon, has never yet been found with an engraving of classic date upon it ; that the steatite of catalogues of engraved gems is for the most part serpentine, a harder mineral having an essentially different constitution ; and that under the conventional term " plasma " several other minerals are included, such as jade and smaragdite, both varieties of hornblende, and even the beautiful rich green variety of serpentine known as antigorite. CHAPTER V. ARTIFICIAL FORMATION OF PRECIOUS STONES. A CLEAR distinction must be made between the imitation of a precious stone and its actual reproduction or formation by arti- ficial methods. In the former case we simulate the appearance of the natural substance by means of some product or prepara- tion, which may be (and generally is) widely different in chemical composition and even in many physical properties. In the latter case we form the very mineral which Nature has formed, endowed with all its chemical and physical characters, but not necessarily produced by processes identical with those of Nature. A few examples of the true reproduction of precious stones will serve to explain the distinction pointed out with sufficient exactitude. Take the case of the ruby and sapphire, varieties of crystal- lized alumina or corundum. If, by the aid of the intense heat of the oxyhydrogen blowpipe pure alumina, with traces of chromium oxide or other colouring oxide, be fused, we get a sapphire or ruby glass, having a hardness and density much less than those belonging to crystals of alumina. But by prolonging the time of cooling or by producing the alumina from some of its compounds during the heating, a portion of the product will crystallise in forms identical with those of the natural stone, and having the density of 4 and the hardness 9. For some time the specimens made were small in size and poor in colour and brilliancy, but the product was identical with native corundum. Now there have been, among the large numbers of artificially-prepared rubies, some of several carats in ARTIFICIAL FORMATION. 49 weight which can be distinguished from the' natural stones only by a close examination with the microscope. It is then observed that the artificial rubies contain cavities of a different outline and nature to those which occur in the rubies made in nature's laboratory. These cavities are more or less spherical or pear-shaped in the artificial ruby and their walls are curved ; in the natural stone the cavities are really negative crystals while their walls are angular. It was the French chemist Fremy who first made small rubies arti- ficially, but subsequent workers, by employing larger quantities of material, modifying the ingredients taken, and allowing the fused product to cool more slowly, have achieved greater success. The exact way in which the so-called re-constituted rubies are made is not known, nor can absolute reliance be placed upon the descrip- tions which have been given of other methods of ruby-formation. The red spinel has also been made artificially, of good colour, and in large crystals. The spinel is a compound of alumina and magnesia, and by the aid of a substance such as boracic acid, which acts as a solvent for the constituents of spinel, but which volatilizes at very high temperatures, crystals of spinel having considerable dimen- sions, good colours, and the hardness of 8, have been obtained by several chemists. These stones, having been cut and polished, could not be distinguished by any test from the natural gems. Another method of operating, by which rock crystal and a con- siderable number of hard transparent and beautiful compounds of silica have been made, consists in causing two substances to act upon each other when both are in the state of vapour, sometimes with the aid of the vapour of water as a decomposing agent, and sometimes without. By the reaction of fluoride of aluminium and boracic acid, fluoride of boron and alumina are produced, the latter crystallizing in colourless rhombohedra of white sapphire, or even, when chromium is added, taking the colours of ruby and blue sapphire. Similarly treated at a very high temperature in a lime crucible, the fluorides of aluminium and glucinum have been made 8445. D 5 o PRECIOUS STONES. to yield distinct crystals of chrysoberyl. It is probable that in Nature the formation of gem-stones has occurred in the presence of water, and under very great pressure continued for a long time. Indeed, it may be concluded that the agency of a very high tempera- ture has not been generally at work, but that the important elements in the production of natural crystals have been time, mass, and pressure. Chemists have devoted much time and skill to devising methods for producing diamonds. These methods have rarely been suc- cessful, but M. Henri Moissan has really made a number of small very small diamonds by causing carbon to dissolve in molten iron at the high temperature of the electric furnace and then, by sudden cooling of the metallic mass, causing the formation of a rigid shell and so producing great pressure in the interior : the iron mass treated with acid left a residue containing small diamond crystals : by slow cooling graphite only was formed. CHAPTER VI. IMITATIONS OF PRECIOUS STONES. THE one point in which all artificial imitations of precious stones fail is hardness. Practically they all yield to the file, and many are scratched even by a bit of common glass. Indeed, with rare exceptions, they consist of flint glass containing an unusually large proportion of lead and tinctured by the addition of certain colour- ing oxides, such as cobalt for blue, manganese for violet, as well as nickel, copper, iron, chromium, or mixtures of these, for other hues. Colourless s trass, as it is called, commonly contains 38 per cent, of silica, 53 oxide of lead, 8 potash, and traces of boracic and arsenious acid, with some alumina and soda. There are three other points in which these coloured glasses differ from true stones. Besides their softness already named, they tarnish in impure air, the lead becom- ing sulphided, and therefore brown ; they are heavier than any of the stones having specific gravity under 3*3, which they represent, and they are all destitute of pleochroism. Under the microscope, or even a hand magnifier, the majority of them show many lines, and specks, and air-bubbles, which betray their origin and nature their origin, at a high temperature rapidly reduced ; their nature, as fused, glassy, non- crystalline masses. The lines and striae are signs of layers of unequal density and of strain ; the bubbles are rounded cavities, quite different from those cavities, with angular and crystalline walls, which some gem-stones, such as amethyst, beryl, topaz, frequently present. This is true not only of the many varieties of coloured paste or " strass," which form the usual 8445. J 2 52 PRECIOUS STONES. materials for imitative gems, but also of the fused compounds having the precise (or at least analogous) chemical composition of various gem-stones which have been prepared by Mr. Grsville Williams and M. Feil. The green beryl glass of the former, and the blue lime spinel of the latter, afford cases in point. Instead of substituting a wholly imitative preparation for a true stone, a doublet or triplet is constructed, in which a colour- less or pale stone, of no value, is made to appear possessed of a fine deep colour. The doublet sapphire has a table and crown all the stone down to the girdle of colourless or pale blue sapphire, then the lower part of the combination, attached by cement, is made from blue glass or strass. If then the upper part of the stone be tested for hardness it answers to that of the sapphire, but if the base be examined, it immediately betrays its softness. To avoid this the triplet has been devised. Here we have pale sapphire for crown and base, but a thin layer of deep blue 'glass at the girdle a part generally hid by the mount. To detect this imposture immersion in water generally suffices, for then the three layers will become visible ; and if a doublet or triplet be boiled in water, or soaked in a small bottle of chloroform, it usually betrays its composite nature by falling to pieces. We should add that some false stones of this sort are coloured by means of a layer of coloured varnish or cement. Imitation pearls claim a word of description. They are small spheres blown on tubes of slightly opalescent glass, and coated internally with a preparation made from the scales of a certain fish (as the bleak), and called Essence d'Orient. Into the little opalescent glass globe a coating of parchment size is introduced, and then a film of the pearl essence. Lastly, when the essence is dry, the bead is filled with wax. In order to produce an appearance like the orient of the true pearl the glass globes before filling are sometimes heated under pressure with a hydrochloric acid solution ; in tjiis way an iridescent surface effect is produced. IMITATIONS. 53 * Some remarks on the artificial colouring' of natural stones will be found in chapter vii. ; the different varieties of silica agate, onyx, cornelian, and even opal are frequently subjected to processes of heating and saturation with chemical reagents in order to change their hue or to introduce foreign colouring matters. CHAPTER VII. DESCRIPTION OF PRECIOUS STONES. DIAMOND. THERE are three characters which unite to place the diamond in a unique position amongst precious stones. It is the only gem which is combustible ; it is the hardest of all minerals ; it exerts upon light the most energetic refractive and dispersive power. The diamond belongs to the cubic or isometric system, and usually occurs in the form of an octahedron, or in combinations in which the cube, the dodecahedron, and the tetrahedron are involved. The faces of these forms are commonly curved : macled and hemitropic associations of crystals are of frequent occurrence. The diamond is easily cleaved in directions parallel to its octahedral faces. Its fracture is conchoidal. Its hardness is 10. The lustre both of natural and artificial surfaces of diamond is peculiarly brilliant, approaching that of such a metal as silver. This characteristic lustre, which is shared to some extent by sphene, jargoon and garnet, is known as adamantine it lies between the metallic and the resinous lustres. The peculiar brilliancy of diamonds results in part from the total reflection of light from their internal faces when the incident light strikes them at an angle DIAMOND. 55 * greater than 24 13'. Diamond refracts light very strongly the index of refraction for the yellow ray being 2*419, while that of rock crystal is but 1*545; of topaz, 1*621; of white sapphire, i'75; of phenakite, 1*675; and of white zircon, 2. In the extent to which diamond disperses the several coloured rays into which white light is split, this gem greatly surpasses all others. Its " fire," or the flashing of prismatic hues which characterises this precious stone, is mainly due to this dispersive power. The specific gravity of the diamond, when transparent and colour- less, is of remarkable constancy. When taken in the ordinary way, without the refinement of certain small corrections which are made only for scientific purposes, the best results have lain between the narrow limits of 3 * 52 and 3 ' 53, at 60 Fahrenheit. The fine colour- less Porter Rhodes diamond has the specific gravity 3*523 : the smaller but equally fine Gor-do-norr, 3 -527. The former stone was found at Kimberley, South Africa, on the I2th of February, 1881, and weighs 474 troy grains : the latter is of Indian origin, and weighs 213^ grains. The Star of the SoutH, a Brazilian stone of 254! carats, has the specific gravity of 3*529, according to M. Halphen. The range of colour of the diamond is extensive ; but various hues of yellow, grayish yellow, brown, and straw colour, are the most common. Strongly-coloured diamonds are very rare ; but green, blue, and even red stones are known. The celebrated Hope blue diamond, of 44^ carats, and the Brunswick blue diamond, of 6J carats, are both of the same brilliant and steely blue, and may very likely have both been parts of the French blue diamond stolen from the Garde-Meuble, in 1792, and never since seen. The least valuable diamonds are those which lack brilliancy, or have faint hues of gray, brown, and yellow. The most prized are those which combine brilliancy with decided tints of rose, green, or 56 PRECIOUS STONES. blue : cinnamon-coloured, salmon, or puce diamonds are also much esteemed. But pure diamonds, without flaw or tint of any sort, are those which are regarded as coming up to the market standard of excellence, and are spoken of as of the " first water." But even under this designation there is room for considerable diversity of quality, and consequently of price. And there are occasionally met with stones of such exceptional purity and beauty that the ordinary rules of valuation applicable to stones of the " first water " do not hold good. This observation, of course, refers to cut stones, that is, to well-proportioned brilliants. Such a stone, weighing but i carat (3*17 grains), might fetch 30 at a time when a first-water brilliant of the same weight would not realise above 20. In fact, specimen stones, like exceptionally large stones, cannot be said to be amen- able to any precise rule of valuation. The value of the diamond increases in an increasing ratio with its weight up to stones of moderate size, beyond which no rule holds good. Assuming a first-water brilliant of I carat to be worth 20, then an equally fine 2-carat stone would fetch 60, or 30 per carat. Formerly the value of the larger brilliants in- creased so rapidly with their weight that a stone of 10 carats was worth over 200 per carat. But since the South African diamond fields have been extensively worked, large stones have been found in greater abundance, and have not maintained their relatively high prices. In the preceding paragraph brilliants of the first water have been considered, but it should be added that the diamonds used in ordinary shop- jewellery, being either dull, flawed, or " off- colour," possess small market value. Reference may here be made to a trick by which the yellowish hue of a diamond may be temporarily masked. The back facets of the stone are lightly rubbed with a violet-blue wax pencil and the colour distributed by means of a bit of soft paper. The stone is then returned to its DIAMOND. 57 * setting, when it will appear nearly white, the 'blue material correct- ing the yellow hue of the gem. For the localities where diamonds have been or are found reference may be made to the works named in the brief biblio- graphy in the present manual. The story of the diamond- fields of the world is full of romantic interest. India, Brazil, Borneo, and South Africa have all furnished most curious contributions to the long list of adventures, discoveries, and disasters connected with the diamond. Until January 25th. 1905, when the Cullinan diamond weighing 3,032 carats, or 621 J grams, was found in the Premier mine 20 miles W.N.W. of Pretoria, the largest known diamond was that from the Jagersfontein mine, Orange River Colony, discovered on June 3oth, 1893. It weighed 971! carats, but owing to an imperfection the largest brilliant cut from it weighed only 239 carats. It has been called the " Jubilee " and the " Excelsior," and is of high quality. The largest diamond found at Kimberley was an octahedron of 503 carats, but this stone was full of black spots. Amongst the diamonds obtained from the river Vaal diggings the largest is a rounded pebble weighing 330^ carats, but it is not of good quality. It has been estimated that the value of the diamonds added to the world's stock from the South African mines is more than 85,000,000. Even in one year (1903) the value of diamonds ex- ported from Cape Colony was close upon 5,500,000. By the side of these figures the yields in ancient days of India, and, since the year 1725, of Brazil, do not seem large. At the present time diamonds are still found in Brazil, while new sources have been dis- covered in New South Wales, Borneo, and British Guiana. From the last named colony 173,744 stones were exported in 1902, but they were very small, for they weighed altogether no more than 11,518 carats. The winning of diamonds and their mode of occurrence in the 5 8 PRECIOUS STONES. South African diamond fields are fully discussed in the volume of Mr. F. Gardner Williams on " The Diamond Mines of South Africa." Here it must suffice to state that the De Beers' and Kimberley floors, whither the " blue ground " is conveyed, and where it is spread out to weather, cover an area of two thousand acres. Here the blue ground is harrowed, and, if necessary, watered. After various crushing, washing and screening operations, a material is obtained in which the diamonds have become concentrated. This passes at last into a remarkable machine called the Greaser. The mixture of pebbles, which we may call the concentrate, con- tains many minerals other than diamonds, such as garnet, ilmenite, enstatite, chromite, zircon, kyanite, diopside and half-a-dozen other species, varying in density from 2*6 to 5*3. When this mixture flows in a current 'of water on to a series of sloping cast- iron rocking plates covered with a thick layer of grease, the dia- monds adhere to the grease, while the other minerals, both those which are heavier and those which are lighter than diamond, are carried forward and away. Bits of metal and of iron pyrites do get embedded in the grease along with diamonds, and if any corun- dum were present it would also remain, but the separation of these substances from the grease and from the diamonds is quite easy. The grease loses its adhesive power by becoming superficially incorporated with minute portions of water, and then needs remelting and re-spreading on the oscillating " greasers." This discriminating process is based upon the differing surface-attractions of certain minerals for water on the one hand, and for oily and greasy materials on the other. In simpler words diamonds and a few other minerals such as sapphires are apparently more easily oiled than wetted, while the far greater number of minerals are more easily wetted than oiled. The following table gives some particulars concerning a few of the best-known and most important cut diamonds above 100 DIAMOND. 59 carats in weight. The figures quoted are carats, but are probably not in all instances based upon one exact standard NAME. ORIGIN. WEIGHT IN THE ROUGH. WEIGHT WHEN CUT. Nizam .... India _ 277 Excelsior S. Africa. 97 If 239 De Beers of 1888 . S. Africa. 428i 228| Orloff . India 194f Darya-i-nur . . . India . . 186 Victoria .... S. Africa. 457^ 180 Taj-i-mah . India 146 Regent or Pitt India 410 136| Austrian Yellow . India . .. 1334 Star of the South . Brazil 2541 125| Tiffany Yellow * , -,, - S. Africa. . . 125| Stewart .... S. Africa. 288| 120 Koh-i-nur . '. . India W6& Full discussions of the history of these diamonds and of many others will be found in the works named in the Bibliographical Notes. Dr. Max Bauer's " Precious Stones " contains a good set of figures representing most of the celebrated big diamonds of the world. A large uncut Cape stone, given to the British Museum by John Ruskin and named after Bishop Colenso, is a good octahedron of 129! carats. Diamonds and the more valuable of precious stones generally are bought and sold by the weight called a carat. This carat, whatever its precise value, is always considered as divisible into 4 diamond grains, but the subdivisions of the carat are usually ex- pressed by the vulgar fractions, one-fourth, one-eighth, one-twelfth, one-sixteenth, one-twenty-fourth, one-thirty-secondth, and one- sixty-fourth. The origin of the carat is to be sought in certain small hard leguminous seeds, which, when once dry, remain con- stant in weight. The brilliant, glossy, scarlet-and-black seed of Abms precLitorius constitutes the Indian rati, about 2 grains ; the Adenantheni pavonina seed is about 4 grains. The seed of 60 PRECIOUS STONES. the locust-tree, Ceratonia siliquu, weighs on the average 3^ grains, and constitutes, no doubt, the true origin of the carat. The carat is not absolutely of the same value in all countries. Its weight, as used for weighing the diamond and other gem- stones in different parts of the world, is given, in decimals of a gram, by the majority of the authorities, as Madras -2073533 Vienna* .... '20613 Frankfort. . . . '20577 Brazil and Portugal . . '20575 France .... -2055 England .... .'205409 Spain .... -205393 Holland! . '205041 Assuming the gram to correspond to 15*43235 English grains, an English diamond carat will nearly equal 3*17 grains. It is, however, spoken of as being equal to 4 grains, the grains meant being " diamond " grains, and not ordinary troy or avoirdupois grains. Thus a diamond grain is but '7925 of a true grain. In an English troy ounce of 480 grains there are 151^ carats ; and so it will be seen that a carat is not indeed quite 3 * 17 grains, but something like 3*1683168 grains, or less exactly, 3*168 grains. Further, if we accept the value in grains of one gram to be, as stated above, 15*43235, and if there be 151 1 carats in a troy ounce of 480 grains, it will follow that an English diamond carat is* 205304 of a grarn, not '205409, as commonly affirmed. By recalculat- ing the value of the diamond carat, as used in different parts of the world, into its scientific equivalents in the metric system, the weight to four places of decimals will become, according to Mr. Lowis D'A. JacksonJ Turin . . . . . -2135 Persia . -2095 Venice -2071 Austro-Hungary . . . -2061 France '2059 Portugal and Brazil . . -2058 Germany .... -2055 England and British India . -2053 Holland and Russia . . -2051 Turkey -2005 Spain -1999 Java and Borneo . . . '1989 Florence .... -1965 Arabia '1944 Egypt -1917 Bologna -1886 * Schrauf gives '2057. t Schrauf gives -20613. t Modern Metrology, p. 377. CORUNDUM. 61 / On the other hand there would appear to be an arrangement between the diamond merchants of London, Paris, and Amster- dam, by which the uniform value for a diamond carat is fixed at 205 of a gram. This value, which was suggested in 1871, by a syndicate of Parisian jewellers, goldsmiths, and others engaged in the commerce in precious stones, was subsequently (1877) confirmed. But, in spite of all efforts to secure uniformity in the standard by which diamonds are bought and sold, very serious discrepancies in sets of carat weights as turned out by different makers still exist. It may be imagined that the diamond does not lend itself readily to the art of the gem engraver, still several engraved diamonds exist. Of these two signets are preserved in the Royal collection at Windsor. One representing the Prince of Wales plumes was cut for Charles I. when Prince of Wales, the other and more important specimen is the armorial signet-ring of Queen Henrietta Maria. This had found its way into the last Duke of Brunswick's collection and then became the property of the city of Geneva. The late Dr. Drury Fortnum bought it and presented it to Queen Victoria. It was engraved in January 1629 to the order of Charles I. by one Francis Walwyn, who received the sum of 267 for his work and for the cost of the boart used. There are other engraved diamonds, mostly of the seventeenth century, of European work- manship in various museums and Royal treasuries. But neither from the artistic nor mineral ogical standpoint are they of much importance. CORUNDUM. Sapphire, Ruby, and Oriental Amethyst. Next to the diamond in hardness must be placed the many varieties of the species called corundum. This includes the sapphire, the ruby, the oriental amethyst, the oriental topaz, and a whol