A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public Part 2

A sample calculation may help make the matter clearer, so one is appended:

Weight of bottle + stone (outside) = 53.51 carats Weight of bottle + stone (inside) = 52.51 carats ------------ Weight of water displaced = 1.00 carat ------------ Weight of stone = 3.51 carats

Weight of stone 3.51 Specific gravity = --------------- = ---- = 3.51 Sp. g.

Weight of water 1.00

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 a.s.sumed 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 sufficiently accurate 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.

TABLE OF SPECIFIC GRAVITIES OF THE PRINc.i.p.aL GEM MATERIALS

Beryl (Emerald) 2.74 Chrysoberyl (Alexandrite) 3.73 Corundum (Ruby, sapphire, "Oriental topaz") 4.03 Diamond 3.52 Garnet (Pyrope) 3.78 " (Hessonite) 3.61 " (Demantoid, known in the trade as "Olivine") 3.84 " (Almandite) 4.05 Opal 2.15 Peridot 3.40 Quartz (Amethyst, common topaz) 2.66 Spinel (Rubicelle, Balas ruby) 3.60 Spodumene (Kunzite) 3.18 Topaz (precious) 3.53 Tourmaline 3.10 Turquoise 2.82 Zircon, lighter variety 4.20 " heavier variety 4.69

For a more complete and scientific discussion of specific gravity determination see _Gem-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 ill.u.s.trations; or see any physics text-book.

LESSON VI

SPECIFIC GRAVITY DETERMINATIONS

WEIGHING A GEM IN WATER. In the previous lesson it was seen that the ident.i.ty 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 gla.s.s or small beaker of water, just as much water will be displaced as though the vessel were full, and it will be displaced _upward_ as 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 fis.h.i.+ng 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 the _loss of weight in water_, and this equals the _weight of an equal volume of water_, which is precisely what we got by our bottle method.

We now need only divide the weight in air by the loss of weight in water, and we shall have the specific gravity of the stone.

[Ill.u.s.tration: 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 trailing partly 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. (See Fig. 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 = 4.02 carats ----------- Weight of stone (plus wire) in water = 3.32 carats Weight of wire = .30 carat ----------- True weight of stone in water = 3.02 carats ----------- Loss of weight in water = 1.00 carat

Weight of stone 4.02 Specific gravity = --------------- = ---- = 4.02 Loss in water 1.00

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 in Lessons V. and VI. 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 distinguis.h.i.+ng 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 the refractive 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 the liquid,) 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's _Gem-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.

LESSON VII

l.u.s.tER AND OTHER REFLECTION EFFECTS

By the term _l.u.s.ter_ we refer to the manner and degree in which light is reflected from the _surface_ of a material. Surfaces of the same material, but of varying degrees of smoothness would, of course, vary in the vividness of their l.u.s.ter, 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 l.u.s.ter that we speak of it as silky l.u.s.ter, and just as pearl has a pearly l.u.s.ter, so certain gems have peculiar and characteristic l.u.s.ter. The diamond gives us a good example. Most diamond dealers distinguish between real and imitation diamonds at a glance by the character of the l.u.s.ter. 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 l.u.s.ter 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 l.u.s.ter 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 l.u.s.ter.

However, in most cases in the trade there is almost no chance of the unexpected presence of a zircon and the l.u.s.ter test is usually sufficient to distinguish the diamond. (Zircons are strongly doubly refractive, as was said in Lesson III. on Double Refraction, and with a lens the doubling of the back lines may be seen.)

ADAMANTINE l.u.s.tER. The l.u.s.ter of a diamond is called _adamantine_ (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 l.u.s.ter. 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 having adamantine l.u.s.ter of any of the other gems. The green garnet that is called "olivine" in the trade also approaches diamond in l.u.s.ter, hence the name "demantoid," or diamond like, sometimes applied to it.

VITREOUS l.u.s.tER. The other stones nearly all have what is called _vitreous_ l.u.s.ter (literally, gla.s.s like), yet owing to difference of hardness, and consequent minute differences in fineness of surface finish, the keenness of this vitreous l.u.s.ter varies slightly in different stones, and a trained eye can obtain clues to the ident.i.ty of certain stones by means of a consideration of the l.u.s.ter. Garnets, for example, being harder than gla.s.s, take a keener polish, and a glance at a doublet (of which the hard top is usually garnet and the base of gla.s.s) will show that the light is better reflected from the garnet part of the top slope than from the gla.s.s part. This use of l.u.s.ter affords the quickest and surest means of detecting a doublet. One can even tell a doublet inside a show window, although the 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 l.u.s.ter, and then the back l.u.s.ter, in rapid succession, one can tell whether or not the stone is a doublet.

OILY l.u.s.tER. Certain stones, notably the peridot (or chrysolite) and the hessonite (or cinnamon stone), have an oily l.u.s.ter. 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 l.u.s.ter 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 l.u.s.ter alone, but it will pay to spend some spare time in studying the l.u.s.ter 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 l.u.s.ter than the other stone, although both have vitreous l.u.s.ter. Similarly the corundum gems (ruby and sapphire), being even harder than true topaz, take a splendid surface finish and have a very keen vitreous l.u.s.ter.

Turquoise has a dull waxy l.u.s.ter, due to its slight hardness. Malachite, although soft, has, perhaps because of its opacity, a keen and sometimes almost metallic l.u.s.ter.

One may note the l.u.s.ter 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 a certain lack of h.o.m.ogeneity 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 small twin crystal layers that reflect light so as to produce a sort of moonlight-on-the-water appearance _within_ the 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 cut _en cabochon_, show a thin sharp line of light running across the center of the stone (when properly cut with the base parallel 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 l.u.s.ter and other types of reflection effects see _Gem-Stones_, by G. F. Herbert-Smith, Chapter V., pp. 37-39, or _A Handbook of Precious Stones_, M. D. Rothschild, pp. 17, 18.

LESSON VIII

HARDNESS