LESSON IX
HARDNESS—Continued
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 small true topaz crystal. (The pure white topaz of Thomas Mountain, Utah, is excellent; 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 91⁄2 if 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, one may 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 the feel and 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 the use 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 see Gem-Stones, G. F. Herbert-Smith, Chap. IX., pp. 78-81, and table on p. 305; or see A Handbook of Precious Stones, Rothschild, pp. 19, 20, 21.
LESSON X
DISPERSION
Another property which may be made use of in deciding the identity of certain gems is that called dispersion. We have seen in [Lesson II.] that light in entering a stone from the air changes its path (refraction), and in [Lesson 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 light of 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. These spectra, 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—they will 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 within them. 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 they blend 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 suspect glass.
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's Gem-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.