Weight of bottle + stone (outside)=53.51caratsWeight of bottle + stone (inside)=52.51caratsWeight of water displaced=1.00caratWeight of stone=3.51carats
Specific gravity =Weight of stone=3.51= 3.51 Sp. g.Weight of water1.00
In this case the specific gravity being 3.51, the stone is probably diamond (see table), but might be precious topaz, which has nearly the same specific gravity.
It is assumed that the jeweler will weigh in carats, and that his balance is sensitive to .01 carat. With such a balance, and a specific gravity bottle (which any scientific supply house will furnish for less than $1) results sufficientlyaccurate for the determination of precious stones may be had if one is careful to exclude air bubbles from the bottle, and to wipe the outside of the bottle perfectly dry before each weighing. The bottle should never be held in the warm hands, or it will act like a thermometer and expand the water up the narrow tube in the stopper, thus leading to error. A handkerchief may be used to grasp the bottle.
Beryl (Emerald)2.74Chrysoberyl (Alexandrite)3.73Corundum (Ruby, sapphire, "Oriental topaz")4.03Diamond3.52Garnet(Pyrope)3.78"(Hessonite)3.61"(Demantoid, known in the trade as "Olivine")3.84"(Almandite)4.05Opal2.15Peridot3.40Quartz (Amethyst, common topaz)2.66Spinel (Rubicelle, Balas ruby)3.60Spodumene (Kunzite)3.18Topaz (precious)3.53Tourmaline3.10Turquoise2.82Zircon,lighter variety4.20"heavier variety4.69
For a more complete and scientific discussion of specific gravity determination seeGem-Stones, by G. F. Herbert-Smith, Chapter VIII., pp. 63-77; or see,A Handbook of Precious Stones, by M. D. Rothschild, pp. 21-27, for an excellent account with illustrations; or see any physics text-book.
Weighing a Gem in Water.In the previous lesson it was seen that the identity of a precious stone may be found by determining its specific gravity, which is a number that tells how much heavier the material is than a like volume of water. It was not explained, however, how one would proceed to get the specific gravity of a stone too large to go in the neck of a specific gravity bottle. In the latter case we resort to another method of finding how much a like volume of water weighs. If the stone, instead of being dropped into a perfectly full bottle of water (which then overflows), be dropped into a partly filled glass or small beaker of water, just as much water will be displaced as though the vessel were full, and it will be displacedupwardas before, for lack of any other place to go. Consequently its weight will tend to buoy up or float the stone by trying to get back under it, and the stone when in water will weigh less than when in air. Anyone who has ever pulled up a small anchor when out fishing from a boat will recognize at once that this is the case, and that as the anchor emerges from the water it seems to suddenly grow heavier. Not only does the stone weigh less when in the water, but it weighs exactly as much less as the weight of the water that was displaced by the stone (which has a volume equal to the volume of the stone). If we weigh a stone first in the air, as usual, and then in water (where it weighs less), and then subtract the weight in water from the weight in air we will have theloss of weight in water, and this equals theweight of an equal volume of water, which is precisely what we got by our bottle method.
We now need only divide the weight in airby the loss of weight in water, and we shall have the specific gravity of the stone.
Fig.6.
To actually weigh the stone in water we must use a fine wire to support the stone. We must first find how much this wire itself weighs (when attached by a small loop to the hook that supports the balance pan and trailingpartly in the water, as will be the case when weighing the stone in water). This weight of the wire must of course be deducted to get the true weight of the stone in water. The beaker of water is best supported by a small table that stands over the balance pan. One can easily be made out of the pieces of a cigar box. (SeeFig. 6.)
The wire that is to support the stone should have a spiral at the bottom in which to lay the gem, and this should be so placed that the latter will be completely submerged at all times, but not touching bottom or sides of the beaker.
Example of data, and calculation, when getting specific gravity by the method of weighing in water:
Weight of stone=4.02caratsWeight of stone (plus wire) in water=3.32caratsWeight of wire=.30caratTrue weight of stone in water=3.02caratsLoss of weight in water=1.00carat
Specific gravity =Weight of stone=4.02= 4.02Loss in water1.00
Here the specific gravity, 4.02 would indicate some corundum gem (ruby or sapphire), and the other characters would indicate at once which it was.
The student who means to master the use of the two methods given inLessons V.andVI.should proceed to practice them with stones of known specific gravities until he can at least get the correct result to the first decimal place. It is not to be expected that accurate results can be had in the second decimal place, with the balances usually available to jewelers. When the learner can determine specific gravities with some certainty he should then try unknown gems.
The specific gravity method is of especial value in distinguishing between the various colorless stones, as, for example, quartz crystal, true white topaz, white sapphire, white or colorless beryl, etc. These are all doubly refractive, have no color, and hence no dichroism, and unless one has a refractometer to get therefractive index, they are difficult to distinguish. The specific gravities are very different, however, and readily serve to distinguish them. It should be added that the synthetic stones show the same specific gravities as their natural counterparts, so that this test does not serve to detect them.
Where many gems are to be handled and separated by specific gravity determinations, perhaps the best way to do so is to have several liquids of known specific gravity and to see what stones will float and what ones will sink in the liquids. Methylene iodide is a heavy liquid (sp. g. 3.32), on which a "quartz-topaz," for example, sp. g. 2.66, would float, but a true topaz, sp. g. 3.53, would sink in it. By diluting methylene iodide with benzol (sp. g. 0.88) any specific gravity that is desired may be had (between the two limits 0.88 and 3.32). Specimens of known specific gravity are used with such liquids and their behavior (as to whether they sink or float, or remain suspended in theliquid,) indicates the specific gravity of the liquid. An unknown stone may then be used and its behavior noted and compared with that of a known specimen, whereby one can easily find out whether the unknown is heavier or lighter than the known sample.
An excellent account of the detail of this method is given in G. F. Herbert-Smith'sGem-Stones, pages 64-71, of Chapter VIII., and various liquids are there recommended. It is doubtful if the practical gem dealer would find these methods necessary in most cases. Where large numbers of many different unknown gems have to be determined it would pay to prepare, and standardize, and use such solutions.
Bythe termlusterwe refer to the manner and degree in which light is reflected from thesurfaceof a material. Surfaces of the same material, but of varying degrees of smoothness would, of course, vary in the vividness of their luster, but the type of variation that may be made use of to help distinguish gems, depends upon the character of the material more than upon the degree of smoothness of its surface. Just as silk has so typical a luster that we speak of it as silky luster, and just as pearl has a pearly luster, so certain gems have peculiar and characteristic luster. The diamond gives us a good example. Most diamond dealers distinguish between real and imitation diamonds at a glance by the characterof the luster. That is the chief, and perhaps the only property, that they rely upon for deciding the genuineness of a diamond, and they are fairly safe in so doing, for, with the exception of certain artificially decolorized zircons, no gem stone is likely to deceive one who is familiar with the luster of the diamond. It is not to be denied that a fine white zircon, when finely cut, may deceive even one who is familiar with diamonds. The author has fooled many diamond experts with an especially fine zircon, for the luster of zircon does approach, though it hardly equals, that of the diamond. Rough zircons are frequently mistaken for diamonds by diamond prospectors, and even by pickers in the mines, so that some care should be exercised in any suspicious case, and one should not then rely solely on the luster. However, in most cases in the trade there is almost no chance of the unexpected presence of a zircon and the luster test is usually sufficient to distinguish the diamond. (Zircons are strongly doublyrefractive, as was said inLesson III.on Double Refraction, and with a lens the doubling of the back lines may be seen.)
Adamantine Luster.The luster of a diamond is calledadamantine(the adjective uses the Greek name for the stone itself). It is keen and cold and glittering, having a metallic suggestion. A very large per cent. of the light that falls upon the surface of a diamond at any low angle is reflected, hence the keenness of its luster. If a diamond and some other white stone, say a white sapphire, are held so as to reflect at the same time images of an incandescent light into the eye of the observer, such a direct comparison will serve to show that much more light comes to the eye from the diamond surface than from the sapphire surface. The image of the light filament, as seen from the diamond, is much keener than as seen from the sapphire. The same disparity would exist between the diamond and almost any other stone. Zircon comes nearest to havingadamantine luster of any of the other gems. The green garnet that is called "olivine" in the trade also approaches diamond in luster, hence the name "demantoid," or diamond like, sometimes applied to it.
Vitreous Luster.The other stones nearly all have what is calledvitreousluster (literally, glass like), yet owing to difference of hardness, and consequent minute differences in fineness of surface finish, the keenness of this vitreous luster varies slightly in different stones, and a trained eye can obtain clues to the identity of certain stones by means of a consideration of the luster. Garnets, for example, being harder than glass, take a keener polish, and a glance at a doublet (of which the hard top is usually garnet and the base of glass) will show that the light is better reflected from the garnet part of the top slope than from the glass part. This use of luster affords the quickest and surest means of detecting a doublet. One can even tell a doublet inside a show window, althoughthe observer be outside on the sidewalk, by moving to a position such that a reflection from the top slope of the stone is to be had. When a doublet has a complete garnet top no such direct comparison can be had, but by viewing first the top luster, and then the back luster, in rapid succession, one can tell whether or not the stone is a doublet.
Oily Luster.Certain stones, notably the peridot (or chrysolite) and the hessonite (or cinnamon stone), have an oily luster. This is possibly due to reflection of light that has penetrated the surface slightly and then been reflected from disturbed layers beneath the surface. At any rate, the difference in luster may be made use of by those who have trained their eyes to appreciate it. Much practice will be needed before one can expect to tell at a glance when he has a peridot (or chrysolite) by the luster alone, but it will pay to spend some spare time in studying the luster of the various stones.
A true, or "precious" topaz, for example,may be compared with a yellow quartz-topaz, and owing to the greater hardness of the true topaz, it will be noted that it has a slightly keener luster than the other stone, although both have vitreous luster. Similarly the corundum gems (ruby and sapphire), being even harder than true topaz, take a splendid surface finish and have a very keen vitreous luster.
Turquoise has a dull waxy luster, due to its slight hardness. Malachite, although soft, has, perhaps because of its opacity, a keen and sometimes almost metallic luster.
One may note the luster rapidly, without apparatus and without damage to the stone. We thus have a test which, while it is not conclusive except in a very few cases, will supplement and serve to confirm other tests, or perhaps, if used at first, will suggest what other tests to apply.
Another optical effect that serves to distinguish some stones depends upon the reflection of light from within the material due to acertain lack of homogeneity in the substance.
Cause of Color in the Opal.Thus the opal is distinguished by the prismatic colors that emerge from it owing to the effect of thin layers of material of slightly different density, and hence of different refractive index from the rest of the material. These thin films act much as do soap-bubble films, to interfere with light of certain wave lengths, but to reflect certain other wave lengths and hence certain colors.
Again, in some sapphires and rubies are found minute, probably hollow, tube-like cavities, arranged in three sets in the same positions as the transverse axes of the hexagonal crystal. The surfaces of these tubes reflect light so as to produce a six-pointed star effect, especially when the stone is properly cut to a high, round cabochon form, whose base is parallel to the successive layers of tubes.
Starstones, Moonstones, Cat's-eyes.In the moonstone we have another sort of effect, this time due to the presence of hosts of smalltwin crystal layers that reflect light so as to produce a sort of moonlight-on-the-water appearancewithinthe stone when the latter is properly cut, with the layers of twin crystals parallel to its base. Ceylon-cut moonstones are frequently cut to save weight, and may have to be recut to properly place the layers so that the effect may be seen equally over all parts of the stone, as set.
Cat's-eye and tiger's-eye owe their peculiar appearance to the presence, within them, of many fine, parallel, silky fibers. The quartz cat's-eye was probably once an asbestos-like mineral, whose soft fibers were replaced by quartz in solution, and the latter, while giving its hardness to the new mineral, also took up the fibrous arrangement of the original material. The true chrysoberyl cat's-eye also has a somewhat similar fibrous or perhaps tubular structure. Such stones, when cuten cabochon, show a thin sharp line of light running across the center of the stone (when properly cut with the baseparallel to the fibers). This is due to reflection of light from the surfaces of the parallel fibers. The line of light runs perpendicularly to the fibers.
In these cases (opals, starstones, moonstones, and cat's-eyes) the individual stone is usually easily distinguished from other kinds of stones by its peculiar behavior towards light. However, it must be remembered that other species than corundum furnish starstones (amethyst and other varieties of quartz, for example), so that it does not follow that any starstone is a corundum gem. Also the more valuable chrysoberyl cat's-eye may be confused with the cheaper quartz cat's-eye unless one is well acquainted with the respective appearances of the two varieties. Whenever there is any doubt other tests should be applied.
For further account of luster and other types of reflection effects seeGem-Stones, by G. F. Herbert-Smith, Chapter V., pp. 37-39, orA Handbook of Precious Stones, M. D. Rothschild, pp. 17, 18.
Anotherproperty by means of which one may distinguish the various gems from each other ishardness. By hardness is meant the ability to resist scratching. The term "hardness" should not be taken to include toughness, yet it is frequently so understood by the public. Most hard stones are more or less brittle and would shatter if struck a sharp blow. Other hard stones have a pronouncedcleavageand split easily in certain directions. True hardness, then, implies merely the ability to resist abrasion (i. e., scratching).
Now, not only is hardness very necessary in a precious stone in order that it mayreceiveandkeepa fine polish, but the degree in which it possesses hardness as comparedwith other materials of known hardness may be made use of in identifying it.
No scale ofabsolutehardness has ever come into general use, but the mineralogist Mohs many years ago proposed the followingrelativescale, which has been used very largely:
Mohs's Scale of Hardness.Diamond, the hardest of all gems, was rated as 10 by Mohs. This rating was purely arbitrary. Mohs might have called it 100 or 1 with equal reason. It was merely in order to represent the different degrees of hardness by numbers, that he picked out the number 10 to assign to diamonds. Sapphire (and ruby) Mohs called 9, as being next to diamond in hardness. True topaz (precious topaz) he called 8. Quartz (amethyst and quartz "topaz") was given the number 7. Feldspar (moonstone) was rated 6, the mineral apatite 5, fluorspar 4, calcite 3, gypsum 2, and talc 1.
It may be said here that any mineral in this series, that is of higher number thanany other, will scratch the other. Thus diamond (10) will scratch all the others, sapphire (9) will scratch any but diamond, topaz (8) will scratch any but diamond and sapphire, and so on.
It must not be thought that there is any regularity in the degrees of hardness as expressed by these numbers. The intervals in hardness are by no means equal to the differences in number. Thus the interval between diamond and sapphire, although given but one number of difference, is probably greater than that between sapphire (9) and talc (1). The numbers thus merely give us an order of hardness. Many gem minerals are, of course, missing from this list, and most of the minerals from 5 down to 1 are not gem minerals at all. Few gem materials are of less hardness than 7, for any mineral less hard than quartz (7) will inevitably be worn and dulled in time by the ordinary road dust, which contains much powdered quartz.
In testing a gem for hardness the problem consists in finding out which of the above minerals is most nearly equal in hardness to the unknown stone. Any gem that was approximately equal in hardness to a true topaz (8) would also be said to be of hardness 8. Thus spinel is of about the same hardness as topaz and hence is usually rated as 8 in hardness. Similarly opal, moonstone, and turquoise are of about the same hardness as feldspar and are all rated 6.
Frequently stones will be found that in hardness are between some two of Mohs's minerals. In that case we add one half to the number of the softer mineral; thus, peridot, benitoite, and jade (nephrite) are all softer than quartz (7) but harder than feldspar (6); hence we say they are 61⁄2in hardness. Beryl (aquamarine and emerald), garnet (almandine), and zircon are rated 71⁄2in hardness, being softer than true topaz but harder than quartz. A table of the hardness of most ofthe commonly known gem-stones follows this lesson.
Having now an idea of what hardness means and how it is expressed, we must next inquire how one may make use of it in identifying unknown gems.
How to Apply the Hardness Test.In the first place, it is necessary to caution the beginner against damaging a fine gem by attempting to test its hardness in any but the most careful manner. The time-honored file test is really a hardness test and serves nicely to distinguish genuine gems, of hardness 7 or above, from glass imitations. A well-hardened steel file is of not quite hardness 7, and glass of various types while varying somewhat averages between 5 and 6. Hence, glass imitations are easily attacked by a file. To make the file test use only avery finefile and apply it with a light but firm pressure lengthwise along the girdle (edge) of the unset stone. If damage results it will then be almost unnoticeable.Learn to know thefeelof the file as it takes hold of a substance softer than itself. Also learn thesound. If applied to a hard stone a file will slip on it, as a skate slips on ice. It will not take hold as upon a softer substance.
If the stone is set, press a sharp corner of a broken-ended file gently against abackfacet, preferably high up toward the girdle, where any damage will not be visible from the front, and move the file very slightly along the surface, noting by thefeelwhether or not it takes hold and also looking with a lens to see if a scratch has been made. Do not mistake a line of steel, left on a slightly rough surface, for a true scratch. Frequently on an unpolished girdle of real gem material the file will leave a streak of steel. Similarly when using test minerals in accordance with what follows do not mistake a streak of powder from the yielding test material, for a true scratch in the material being tested. The safe way is to wipe the spotthus removing any powder. A true scratch will, of course, persist.
A doublet, being usually constructed of a garnet top and a glass back, may resist a file at the girdle if the garnet top covers the stone to the girdle, as is sometimes the case, especially in the smaller sizes. In this case the back must be tested.
One should never pass a file rudely across the corners or edges of the facets on any stone that may be genuine, as such treatment really amounts to a series of light hammer blows, and the brittleness of most gem stones would cause them to yield, irrespective of their hardness. It should be remembered that some genuine stones are softer than a file, so that it will not do to reject as worthless any material that is attacked by a file. Lapis lazuli (5), sphene (5), opal (6), moonstone (6), amazonite (6), turquoise (6), peridot (61⁄2), demantoid garnet (61⁄2) (the "olivine" of the trade), and jade(nephrite) (61⁄2), are all more or less attacked by a file.
10.Diamond.91⁄2.(Carborundum.)9.Sapphire and ruby (also all the color varieties of sapphire).81⁄2.Chrysoberyl (alexandrite).8.True topaz and spinel (rubicelle, balas ruby).71⁄2.Emerald, aquamarine, beryl, Morganite, zircon (jacinth and true hyacinth and jargoon), almandine garnet.71⁄4.Pyrope garnet (Arizona ruby, cape ruby), hessonite garnet (cinnamon stone), tourmaline (various colors vary from 7 to 71⁄2), kunzite (7+).7.Amethyst, various quartz gems, quartz "topaz," jade (jadeite).61⁄2.Peridot (chrysolite), demantoid garnet ("olivine"), jade (nephrite).6.Opal, moonstone, turquoise.5.Lapis lazuli.
Minerals Used in Testing Hardness.For testing stones that are harder than a file the student should provide himself with the following set of materials:
1. A small crystal of carborundum. (Most hardware stores have specimen crystals as attractive advertisements of carborundum as an abrasive material, or the Carborundum Co., Niagara Falls, N. Y., will supply one.)
2. A small crystal of sapphire (not of gem quality, but it should be transparent and compact. A pale or colorless Montana sapphire can be had for a few cents of any mineral dealer).
3. A smalltrue topazcrystal. (The pure white topaz of Thomas Mountain, Utah, isexcellent; or white topaz from Brazil or Japan or Mexico or Colorado will do. Any mineral house can furnish small crystals for a few cents when not of specially fine crystallization.)
4. A small quartz crystal. (This may be either amethyst or quartz-topaz or the common colorless variety. The fine, sharp, colorless crystals from Herkimer County, N. Y., are excellent. These are very inexpensive.)
5. A fragment of a crystal of feldspar. (Common orthoclase feldspar, which is frequently of a brownish pink or flesh color, will do.)
These five test stones represent the following degrees of hardness:
1. Carborundum is harder than any gem material but diamond. It will scratch sapphire and ruby, which are rated 9 in hardness, hence we may call carborundum 91⁄2if we wish. It is, however, very much softer than diamond, and the latter will scratch it upon the slightest pressure.
2. Sapphire, of hardness 9, scratching any gem material except diamond.
3. True topaz, of hardness 8. It is scratched by sapphire (and, of course, ruby), also by chrysoberyl (which is hence rated 81⁄2), but scratches most other stones. Spinel (which is also rated as 8 in hardness) is really a bit harder than topaz.
4. Quartz, of hardness 7, and scratched by all the previous stones but scratching those that were listed above as of less hardness than a file.
5. Feldspar, of hardness 6, hence slightly softer than a file and yielding to it, but scratching the stones likewise rated as 6 when applied forcibly to them. Also scratching stones rated as less than 6 on slight pressure.
We must next consider how these minerals may be safely used upon gem material. Obviously it would be far safer to use them upon rough gem material than upon cut stones. However, with care and some little skill, onemay make hardness tests without particular danger to fine cut material.
The way to proceed is to apply the cut stone (preferably its girdle, or if that is so set as not to be available, a corner where several facets meet) gently to the flat surface of one of the softer test stones, drawing it lightly along the surface and noting thefeeland looking to see if a scratch results. If the test stone is scratched try the next harder test stone similarly.Do not attempt to use the test stone upon any valuable cut stone.Proceed as above until the gem meets a test stone that it does not attack. Its hardness is then probably equal to the latter and perhaps if pressed forcibly against it a slight scratch would result, but it is not advisable to resort to heavy pressure. A light touch should be cultivated in this work. Having now an indication as to the hardness of the unknown gem look up in the table of the previous lesson those gems of similar hardness and then by theuse of some of the tests already given decide which of the stones of that degree of hardness you have.Never rely upon a single test in identifying a gem.
For further study of hardness and its use in testing gems seeGem-Stones, G. F. Herbert-Smith, Chap. IX., pp. 78-81, and table on p. 305; or seeA Handbook of Precious Stones, Rothschild, pp. 19, 20, 21.
Anotherproperty which may be made use of in deciding the identity of certain gems is that calleddispersion. We have seen inLesson II.that light in entering a stone from the air changes its path (refraction), and inLesson III.it was explained that many minerals cause light that enters them, to divide and proceed along two different paths (double refraction). Now it is further true that light of the various colors (red, orange, yellow, green, blue, and violet) is refracted variously—the violet being bent most sharply, the red least, and the other colors to intermediate degrees. The cut (Fig. 7) represents roughly and in an exaggerated manner the effect we are discussing.
Fig.7.
Now in a cut stone this separation of lightof different colors, or dispersion of light, as it is called, results in the reflection of each of the colors separately from the steep sloping back facets of the stone. If almost any clear, colorless facetted stone is placed in the sunlight and a card held before it to receive the reflections, it will be seen that rainbow-like reflections appear on the card. Thesespectra, as they are called, are caused by the dispersion of light. With a diamond the spectra will be very brilliant and of vivid coloring, and the red will be widely separated from the blue. With white sapphire or white topaz, or with rock crystal (quartz), the spectra will be less vivid—theywill appear in pairs (due to the double refraction of these minerals), and the red and blue will be near together (i. e., the spectra will be short). This shortness in the latter cases is due to the small dispersive power of the three minerals mentioned. Paste (lead glass) gives fairly vivid spectra, and they are single like those from diamond, as glass is singly refracting. The dispersion of the heavy lead glass approaches that of diamond. The decolorized zircon (jargoon) has a dispersion well up toward that of diamond and gives fairly vivid spectra on a card, but they are double, as zircon is doubly refracting. Sphene (a gem rarely seen in the trade) and the demantoid garnet (a green gem often called "olivine" in the trade) both have very high dispersive power, exceeding the diamond in this respect. As they are both colored stones (sphene is usually yellowish, sometimes greenish or brown), the vividness of their color-play is much diminished by absorption of light withinthem. So also the color-play of a deeply colored fancy diamond is diminished by absorption.
Dispersion as a Test of the Identity of a Gem.We may now consider how an acquaintance with the dispersive powers of the various stones can be used in distinguishing them. If a stone has high dispersive power it will exhibit "fire," as it is called—i. e., the various colors will be so widely separated within the stone, and hence reflected out so widely separated, that they will fall on the eye (as on the card above) in separate layers, and vivid flashes of red or yellow or other colors will be seen. Such stones as the white sapphire (and others of small dispersion), however, while separating the various colors appreciably as seen reflected on a card, do not sufficiently separate them to produce the "fire" effect when the light falls on the eye. This is because the various colors, being very near together in this case, cross the eye so rapidly, when the stone is moved, that theyblend their effect and the eye regards the light that thus falls upon it as white. We have here a ready means of distinguishing the diamond from most other colorless gems. The trained diamond expert relies (probably unconsciously) upon the dispersive effect (or "fire") nearly as much as upon the adamantine luster, in telling at a glance whether a stone is or is not a diamond. Of all colorless stones, the only one likely to mislead the expert in this respect is the whitened zircon (jargoon), which has almost adamantine luster and in addition nearly as high dispersive power as diamond. However, zircon is doubly refracting (strongly so), and the division of the spectra which results (each facet producing two instead of only one) weakens the "fire" so that even the best zircon is a bit "sleepy" as compared with even an ordinary diamond.
In addition to providing a ready means of identifying the diamond, a high degree of dispersion in a stone of pronounced color would lead one to consider sphene, demantoid garnet(if green), and zircon (which might be reddish, yellowish, brown, or of other colors), and if the stone did not agree with these in its other properties one should suspectglass.
A good way to note the degree of dispersion, aside from the sunlight-card method, is to look at the stone from the back while holding it up to the light (daylight). Stones of high dispersive power will display vivid color play in this position. Glass imitations of rubies, emeralds, amethysts, etc., will display altogether too much dispersion for the natural gems.
In Chap. III., p. 20, of G. F. Herbert-Smith'sGem-Stones, a brief account of dispersion is given. College text-books on physics also treat of it, and the latter give an account of how dispersion is measured and what is meant by a coefficient of dispersion. Most gem books say little about it, but as we have seen above, a knowledge of the matter can, when supplemented by other tests, be applied practically in distinguishing gems.
Inreserving to the last the property ofcolor, which many dealers in gems use first when attempting to identify a precious stone, I have sought to point out the fact that a determination based solely upon color is very likely to be wrong. So many mineral species are found in so many different colors that to attempt to identify any mineral species by color alone is usually to invite disaster. The emerald, alone among gems, has, when of fine color, a hue that is not approached by any other species. The color of the grass in the springtime fitly describes it. Yet even here the art of man has so closely counterfeited in glass the green of the emerald that one cannot be sure of his stone by color alone. As was suggested earlier in theselessons, the writer has several times recently had occasion to condemn as glass imitations stones for which high prices had been paid as genuine emeralds, those who sold them having relied solely upon a trained eye for color.
Confusion of Gems Due to Similarity of Color.The same tendency to rely upon color causes many in the trade to call all yellow stones "topaz" whether the species be corundum (oriental topaz), true topaz (precious topaz), citrine quartz (quartz topaz), heliodor (yellow beryl), jacinth (yellow zircon), or what not.
Similarly the public calls all red stones ruby. Thus we have "cape ruby" and "Arizona ruby" (pyrope garnet), "spinel ruby" (more properly ruby spinel), "Siam ruby" (very dark red corundum), "Ceylon ruby" (pale pinkish corundum), rubellite (pink tourmaline), and lastly Burmah ruby (the fine blood-red corundum).
While it is true that color, unless skillfully estimated and wisely used in conjunction withother properties, is a most unreliable guide, yet when thus used, it becomes a great help and serves sometimes to narrow down the chase, at the start, to a very few species. To thus make use of it requires an actual acquaintance with the various gem materials, in their usual colors and shades and an eye trained to note and to remember minute differences of tint and shade. The suggestions which follow as to usual colors of mineral species must then be used only with discretion and after much faithful study of many specimens of each of the species.
Let us begin with the beginning color of the visible spectrum, red, and consider how a close study of shades of red can help in distinguishing the various red stones from each other. In the first place we will inquire what mineral species are likely to furnish us with red stones. Omitting a number of rare minerals, we have (1) corundum ruby, (2) garnet of various types, (3) zircon, (4) spinel, (5) tourmaline. These fiveminerals are about the only common species which give us an out-and-out red stone. Let us now consider the distinctions between the reds of these different species. The red of the ruby, whether dark (Siam type), blood red (Burmah type), or pale (Ceylon), is more pleasing usually than the red of any of the other species. Viewed from the back of the stone (by transmitted light) it is still pleasing. It may be purplish, but is seldom orange red. Also, owing to the dichroism of the ruby the red is variable according to the changing position of the stone. It therefore has a certain life and variety not seen in any of the others except perhaps in red tourmaline, which, however, does not approach ruby in fineness of red color.
Red Stones of Similar Shades.The garnet, on the other hand, when of fire-red hue, is darker than any but the Siam ruby. It is also more inclined to orange red or brownish red—and the latter is especially true when the stone is seenagainst the light (by transmitted light). Its color then resembles that of a solution of "iron" such as is given as medicine. The so-called "almandine" garnets (those of purplish-red tint) do not equal the true ruby in brightness of color and when held up to the light show more prismatic colors than the true ruby, owing to the greater dispersion of garnet. The color also lacks variety (owing to lack of dichroism). While a fine garnet may make a fair-looking "ruby" when by itself, it looks inferior and dark when beside a fine ruby. By artificial light, too, the garnet is dark as compared with the true ruby, and the latter shows its color at a distance much more strongly than the garnet.
The red zircon, or true hyacinth, is rare. (Many hessonite garnets are sold as hyacinths in the trade. These are usually of a brownish red.) The red of the hyacinth is never equal to that of the ruby. It is usually more somber, and a bit inclined to a brownish cast. The dispersionof zircon, too, is so large (about 87 per cent. of that of diamond) that some little "color-play" is likely to appear along with the intrinsic color. The luster too is almost adamantine while that of ruby is softer and vitreous. Although strongly doubly refracting, the hyacinth shows scarcely any dichroism and thus lacks variety of color. Hence a trained eye will at once note these differences and not confound the stone with ruby.
Spinels, when red, are almost always more yellowish or more purplish than fine corundum rubies. They are also singly refracting and hence exhibit no dichroism and therefore lack variety of color as compared with true ruby. Some especially fine ones, however, are of a good enough red to deceive even jewelers of experience, and one in particular that I have in mind has been the rounds of the stores and has never been pronounced a spinel, although several "experts" have insisted that it was a scientific ruby. The use of a dichroscope wouldhave saved them that error, for the stone is singly refracting. Spinels are usually clearer and more transparent than garnets and show their color better at a distance or when in a poor light.
Tourmaline of the reddish variety (rubellite) is seldom of a deep red. It is more inclined to be pinkish. The dichroism of tourmaline is stronger than that of ruby and more obvious to the unaided eye. The red of the rubellite should not deceive anyone who has ever seen a fine corundum ruby.
Considering next the stones of yellow color, we have the following species to deal with: (1) diamond, (2) corundum, (3) precious topaz, (4) quartz, (5) beryl, (6) zircon, (7) tourmaline.
Yellow Zircon Resembles Yellow Diamond.Here we have less opportunity to judge of the species by the color than was the case withthe red stones. The diamond, of course, is easy to tell, not by the kind of yellow that it displays, for it varies greatly in that respect, but rather by its prismatic play blended with the intrinsic color. Its luster also gives an immediate clue to its identity. It is necessary, however, to be sure that we are not being deceived by a yellow zircon, for the latter has considerable "fire" and a keen luster. Its strong double refraction and its relative softness, as well as its great density will serve to distinguish it. Of the other yellow stones, the true or precious topaz is frequently inclined to a pinkish or wine yellow and many such stones lose all their yellow (retaining their pink) when gently heated. The so-called "pinked" topazes are thus produced.
The yellow corundum rarely has a color that is at all distinctive. As far as color goes the material might be yellow quartz, or yellow beryl, or yellow zircon, or yellow tourmaline (Ceylon type). Many of theyellowish tourmalines have a decidedly greenish cast (greenish-yellow chrysoberyl might resemble these also). However, in general if one has a yellow stone to determine it will be safer to make specific gravity or hardness tests, or both, before deciding, rather than to rely upon color.
Letus first consider what mineral species are most likely to give us green stones. Omitting the semi-precious opaque or translucent stones we have: