# Precious Stones/Chapter 2: Properties and Discrimination of Precious Stones

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:"
| Surface. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Form. | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | 1. Plane. 2. Curved. |
| Surface. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Lustre. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | 3. Metallic. 4. Adamantine. 5. Resinous. 6. Vitreous. 7. Waxy. 8. Pearly. 9. Silky. |
| | | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | 10. Transparent |
| | | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | 11. Translucent |
| | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Light. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | 12. Opalescent |
| | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | 13. Chatoyant. |
| Substance. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | 14. Opaque |
| | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 15. Iridescent |
| | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Colour. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 16. Monochroic. |
| | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Colour. | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 17. Pleochroic. |
| | | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \ \end{matrix}}\right.}$ | 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 important 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 "refraction," 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 ​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 considerable 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 | Tourmaline | 1·642 |
| Zircon | 1·950 | Heavy flint glass | 1·619 |
| Almandine | 1·810 | Beryl | 1·575 |
| Ruby | 1·779 | Rock crystal | 1·549 |
| Pyrope | 1·750 | Iolite | 1·540 |
| Chrysoberyl | 1·748 | Crown glass | 1·524 |
| Peridot | 1·659 | 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 condition, 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 ​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 1·0. In connection with the refraction of light by precious stones mention should be made of the phenomenon of total internal reflection. This, so conspicuous 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 from M. Jannettaz) will perhaps suffice:
| Diamond | ·044 | Red garnet | ·027 |
| Borosilicate of lead | ·049 | Tourmaline | ·019 |
| Flint glass | .036 | Crown glass | ·015 |
| Blue sapphire | ·029 | Rock crystal | ·014 |
Polarization of Light.—There are several ways in which light may suffer the remarkable change known as polarization. If we ​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 18° 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. ​A sliding cap at one end has a square perforation of about .12 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 instrument so constructed the pleochroism of the vast majority of gemstones 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 dichroism 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. 1 to 9 of the Frontispiece.
​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 volume, the symbol ω is employed. The other image, formed by the extraordinary ray and expressed by the symbol ε, 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, ​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 | Apatite | 5 |
| Sapphire | 9 | Fluorspar | 4 |
| Topaz | 8 | Calcite | 3 |
| Quartz | 7 | Rock Salt or Gypsum | 2 |
| Felspar | 6 | 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 gemstones, yet it is sometimes available even in the case of such specimens when unmounted, the "girdle" of the stone offering a suitable surface for a trial of hardness.
Table of Hardness :
| Diamond | 10.0 | Iolite | 7.3 |
| Sapphire and Ruby | 9.0 | Cinnamon stone | 7.0 |
| Chrysoberyl | 8.5 | Jadeite | 7.0 |
| Spinel | 8.0 | Amethyst | 7.0 |
| Topaz | 8.0 | Peridot | 6.5 |
| Aquamarine | 7.8 | Jade | 6.2 |
| Emerald | 7.6 | Moonstone | 6.3 |
| Zircon | 7.5 | Green garnet | 6.0 |
| Tourmaline | 7.5 | Turquoise | 6.0 |
| Phenakite | 7.5 | Opal | 6.0 |
| Almandine | 7.5 | Lapis-lazuli | 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 15°.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: (1) 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 ​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 diplaces when immersed in water in a small vessel of known capacity. We will now briefly describe these three methods.
1. 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 discoverer 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 substitutes 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 ${\ce {Cd3 W9 B2 O31, 2 H2O + 16aq}}$ 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 1-10th 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 ${\ce {CH 2 I2}}$. 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 temperature, has the density 0.869. On the other hand, the density of methylene iodide may be raised by saturating it with iodoform (${\ce {CHI3}}$) 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 ​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 condition. It ought to be mentioned that the methylene iodide preparations, 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 constant 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:
* Thallium and silver nitrate solution of specific gravity 4·5 maintained at a temperature well above its fusing point.
* Thallium and silver nitrate solution of specific gravity 4·1 maintained at a temperature well above its fusing point.
* Thallium and silver nitrate solution of specific gravity 3·9 at 15° C.
* Thallium and silver nitrate solution of specific gravity 3·5 at 15° C.
* Cadmium boro-tungstate solution of specific gravity 3·28 at 15° C., or methylene iodide slightly diluted with toluene.
* Cadmium boro-tungstate solution of specific gravity 2·67: this is prepared 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 be substituted.
​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:—
| Diamond | sink |
| Topaz | sink |
| Spinel | sink |
| Chrysoberyl | sink |
| Alexandrite | sink |
| Pyrope | sink |
| Demantoid | sink |
| Jadeite | sink |
| Diopside | sink |
| Peridot | sink |
| Chrysolite | sink |
| Beryl | sink, while |
| Emerald | sink, while |
| Turquoise | sink, while |
| Phenakite | sink, while |
| Jade | sink, while |
| Tourmaline | sink, while |
| Spodumene | sink, while |
| Opal | float |
| Moonstone | float |
| Lapis-lazuli | float |
| Iolite | float |
| Amethyst | float |
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 ​dust, evaporation, etc., in suitable stoppered and 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 required 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 1 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 explanation of the principle involved.
| A yellow sapphire weighed in air | 12.896 | grams. |
| A yellow sapphire„weighed in„ water | 9.677 | grams.„ |
| Difference, that is, weight of water displaced | 3.219 | grams.„ |
The proportion will be:
| Wt. water displaced. | Sp. gr. water. | Wt. sapphire. | Sp. gr. sapphire. |
| 3.219 : | 1 = | 12.896 : | x |
| x=4.006 |
$y=w.0012\left({\frac {1}{d}}-0.12\right)$
where w is the observed weight in air of a given substance; d its approximate specific gravity; ·0012 the mean density of atmospheric air; ·12 the reciprocal of the specific gravity of brass, and ​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. 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 following 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 | 1.1294 gram. |
| weight„ in air„ alcohol at 15° C. | 0.8064 gram.„ |
| Alcohol displaced | 0.3230 gram.„ |
The equation will be :
${\frac {1.1294\times .8488}{.323}}=2.9676={\textrm {sp.gr.ofphenakite}}$
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 15°C., affords a convenient substitute for spirit. ​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,
${\frac {w}{w+x-y}}={\text{sp. gr.}}$
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 displace 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, | FOUR AND ABOVE, | THREE AND ABOVE. | | TWO AND ABOVE. | TWO AND ABOVE. |
| Hæmatite | 5·3 | Green garnet | 3·85 | Phenakite | 2·98 |
| Pyrites | 5·2 | Chrysoberyl | 3·76 | Turquoise | 2·75 |
| Zircon | 4·6 | Pyrope | 3·75 | Labradorite | 2·72 |
| Almandine | 4·2 | Hessonite | 3·66 | Beryl | 2·70 |
| Sapphire | 4·0 | Spinel | 3·65 | Amethyst | 2·66 |
| | | Topaz | 3·55 | Rock crystal | 2·65 |
| | | Diamond | 3·52 | Iolite | 2·63 |
| | | Peridot | 3·38 | Moonstone | 2·58 |
| | | Spodumene | 3·20 | Opal | 2·20 |
| | | Tourmaline | 3·10 | | |
​The following brief notes as to the physical or mechanical properties 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, (1) 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, | as on„ | diamond. |
| Resinous, | as on„ | garnet. |
| Vitreous, | as on„ | emerald. |
| Waxy, | as on„ | turquoise. |
| Pearly, | as on„ | moonstone. |
| Silky, | as on„ | crocidolite. |
​Metallic and adamantine lustres are connected with high refractive 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.
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 Refractometer, which is available for all gems of which the index of refraction lies between 1·45 and 1·74. 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—α 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 substituted for α bromonaphthaline, the refractive index of which is only 1·658.
* ↑ If the specific gravity of water at 4° C. be taken as 1, then the specific gravities at higher and lower temperatures will be: 0° ·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
* ↑ M. Jolly's spring balance, as modified by Mr. C. F. Cross, is another useful form of instrument for this purpose.
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