The Chemistry, Properties and Tests of Precious Stones Part 3

The standard stones themselves are much more certain in results than the needles, which latter, though well selected and tempered, are not altogether reliable, especially in the more delicate distinctions of picking out the hardest of certain stones of the same kind, in which cases only the expert judge can decide with exactness. Accurate in this the expert always is, for he judges by the sound and depth of his cut, and by the amount and quality of the powder, often calling the microscope to his aid, so that when the decision is made finally, there is never the least doubt about it.

Rapidly as these tests can be made, they are extremely reliable, and should the stone be of great value, it is also subjected to other unerring tests of extreme severity, any one of which would prove it false, if it chanced to be so, though some stones are manufactured and coloured so cleverly that to all but the expert judge and experienced dealer, they would pa.s.s well for the genuine.

In Mohs's list it will be seen that several stones vary considerably, the opal, for instance, having a degree of hardness from 5-1/2 to 6-1/2 inclusive. All stones differ slightly, though almost all may be said to fit their position in the scale; but in the case of the opal, the difference shown is partly due to the many varieties of the stone, as described in the last chapter.

In applying this test of hardness to a cut gem, it will be noticed that some parts of the same stone seem to scratch more readily than others, such as on a facet at the side, which is often softer than those nearest the widest part of the stone, where the claws, which hold it in its setting, usually come. This portion is called the "girdle," and it is on these "girdle" facets that the scratches are generally made. This variation in hardness is mostly caused by cleavage, these cleavage planes showing a marked, though often but slight, difference in the scratch, which difference is _felt_ rather than seen. In addition to the peculiar _feel_ of a cutting scratch, is the _sound_ of it. On a soft stone being cut by a hard one, little or no sound is heard, but there will form a plentiful supply of powder, which, on being brushed off, reveals a more or less deep incision. But as the stones approach one another in hardness, there will be little powder and a considerable increase in the noise; for the harder are the stones, cutting and being cut, the louder will be the sound and the less the powder. An example of this difference is evident in the cutting of ordinary gla.s.s with a "set" or "glazier's" diamond, and with a nail. If the diamond is held properly, there will be heard a curious sound like a keen, drawn-out "kiss," the diamond being considerably harder than the material it cut.

An altogether different sound is that produced by the scratching of gla.s.s with a nail. In this case, the relative difference in hardness between the two is small, so that the gla.s.s can only be scratched and not "cut" by the nail; it is too hard for that, so the noise is much greater and becomes a screech. Experience, therefore, makes it possible to tell to a trifle, at the first contact, of what the stone is composed, and in which cla.s.s it should be placed, by the mere "feel" of the scratch, the depth of it, the amount and kind of powder it leaves, and above all, by the sound made, which, even in the tiniest scratch, is quite characteristic.

CHAPTER VIII.

PHYSICAL PROPERTIES.

F--SPECIFIC GRAVITY.

The fixing of the specific gravity of a stone also determines its group position with regard to weight; its colour and other characteristics defining the actual stone. This is a safe and very common method of proving a stone, since its specific gravity does not vary more than a point or so in different specimens of the same stone. There are several ways of arriving at this, such as by weighing in balances in the usual manner, by displacement, and by immersion in liquids the specific gravity of which are known. Cork is of less specific gravity than water, therefore it floats on the surface of that liquid, whereas iron, being heavier, sinks. So that by changing the liquid to one lighter than cork, the cork will sink in it as does iron in water; in the second instance, if we change the liquid to one heavier than iron, the iron will float on it as does cork on water, and exactly as an ordinary flat-iron will float on quicksilver, bobbing up and down like a cork in a tumbler of water. If, therefore, solutions of known but varying densities are compounded, it is possible to tell almost to exact.i.tude the specific gravity of any stone dropped into them, by the position they a.s.sume.

Thus, if we take a solution of pure methylene iodide, which has a specific gravity of 3.2981, and into this drop a few stones selected indiscriminately, the effect will be curious: first, some will sink plump to the bottom like lead; second, some will fall so far quickly, then remain for a considerable time fairly stationary; third, some will sink very slowly; fourth, some will be partially immersed, that is, a portion of their substance being above the surface of the liquid and a portion covered by it; fifth, some will float on the surface without any apparent immersion. In the first case, the stones will be much heavier than 3.2981; in the second, the stones will be about 3.50; in the third and fourth instances, the stones will be about the same specific gravity as the liquid, whilst in the fifth, they will be much lighter, and thus a rough but tolerably accurate isolation may be made.

On certain stones being extracted and placed in other liquids of lighter or denser specific gravity, as the case may be, their proper cla.s.sification may easily be arrived at, and if the results are checked by actual weight, in a specific gravity balance, they will be found to be fairly accurate. The solution commonly used for the heaviest stones is a mixture of nitrate of thallium and nitrate of silver. This double nitrate has a specific gravity of 4.7963, therefore such a stone as zircon, which is the heaviest known, will float in it. For use, the mixture should be slightly warmed till it runs thin and clear; this is necessary, because at 60 (taking this as ordinary atmospheric temperature) it is a stiff ma.s.s. A lighter liquid is a mixture of iodide of mercury in iodide of pota.s.sium, but this is such an extremely corrosive and dangerous mixture, that the more common solution is one in which methylene iodide is saturated with a mixture of iodoform until it shows a specific gravity of 3.601; and by using the methylene iodide alone, in its pure state, it having a specific gravity of 3.2981, the stones to that weight can be isolated, and by diluting this with benzole, its weight can be brought down to that of the benzole itself, as in the case of Sonstadt's solution. This solution, in full standard strength, has a specific gravity of 3.1789, but may be weakened by the addition of distilled water in varying proportions till the weight becomes almost that of water.

Knowing the specific gravity of all stones, and dividing them into six groups, by taking a series of standard solutions selected from one or other of the above, and of known specific gravity, we can judge with accuracy if any stone is what it is supposed to be, and cla.s.sify it correctly by its mere floating or sinking when placed in these liquids.

Beginning then with the pure double nitrate of silver and thallium, this will isolate the stones of less specific gravity than 4.7963, and taking the lighter solutions and standardising them, we may get seven solutions which will isolate the stones as follows:--

A {shows the stones which have} 4.7963 {a specific gravity over} B " " " 3.70 and under 4.7963 C " " " 3.50 " 3.70 D " " " 3.00 " 3.50 E " " " 2.50 " 3.00 F " " " 2.00 " 2.50 G " " -- -- under 2.00

Therefore each liquid will isolate the stones in its own group by compelling them to float on its surface; commencing with the heaviest and giving to the groups the same letters as the liquids, it is seen that--

_Group_ A.--Isolates gems with a specific gravity of 4.7963 and over 4.70; in this group is placed zircon, with a specific gravity of from 4.70 to 4.88.

_Group_ B.--Stones whose specific gravity lies between 3.70 and under 4.7963.

Garnets, many varieties. See Group D below.

Almandine 4.11 and occasionally to 4.25 Ruby 4.073 " 4.080 Sapphire 4.049 " 4.060 Corundum 3.90 " 4.16 Cape Ruby 3.861 Demantoid 3.815 Staurolite 3.735 Malachite 3.710 and occasionally to 3.996

_Group_ C.--Stones whose specific gravity lies between 3.50 and under 3.70.

Pyrope (average) 3.682 Chrysoberyl 3.689 and occasionally to 3.752 Spinel 3.614 " 3.654 Kyanite 3.609 " 3.688 Hessonite 3.603 " 3.651 Diamond 3.502 " 3.564 Topaz 3.500 " 3.520

_Group_ D.--Stones whose specific gravity lies between 3 and under 3.50.

Rhodonite 3.413 and occasionally to 3.617 Garnets 3.400 " 4.500 Epidote 3.360 " 3.480 Sphene 3.348 and occasionally to 3.420 Idocrase 3.346 " 3.410 Olivine 3.334 " 3.368 Chrysolite 3.316 " 3.528 Jade 3.300 " 3.381 Jadeite 3.299 Axinite 3.295 Dioptase 3.289 Diopside 2.279 Tourmaline (yellow) 3.210 Andalusite 3.204 Apat.i.te 3.190 Tourmaline (Blue and Violet) 3.160 Tourmaline (Green) 3.148 " (Red) 3.100 Spodumene 3.130 and occasionally to 3.200 Euclase 3.090 Fluorspar 3.031 and occasionally to 3.200 Tourmaline (Colourless) 3.029 Tourmaline (Blush Rose) 3.024 Tourmaline (Black) 3.024 and occasionally to 3.300 Nephrite 3.019

_Group_ E.--Stones whose specific gravity lies between 2.50 and under 3.000.

Phenakite 2.965 Turquoise 2.800 Beryl 2.709 and occasionally to 2.81 Aquamarine 2.701 " 2.80 Labradorite 2.700 Emerald 2.690 Quartz 2.670 Chrysoprase 2.670 Jasper 2.668 Amethyst 2.661 Hornstone 2.658 Citrine 2.658 Cordierite 2.641 Agate 2.610 Chalcedony 2.598 and occasionally to 2.610 Adularia 2.567 Rock-crystal 2.521 and occasionally to 2.795

_Group_ F.--Stones whose specific gravity lies between 2.00 and under 2.50.

Hauynite 2.470 and occasionally to 2.491 Lapis lazuli 2.461 Moldavite 2.354 Opal 2.160 and according to variety to 2.283 " (Fire Opal) 2.210 (average)

_Group_ G.--Stones whose specific gravity is under 2.00.

Jet 1.348 Amber 1.000

(See also list of stones, arranged in their respective colours, in Chapter XII.)

In many of these cases the specific gravity varies from .11 to .20, but the above are the average figures obtained from a number of samples specially and separately weighed. In some instances this difference may cause a slight overlapping of the groups, as in group C, where the chrysoberyl may weigh from 3.689 to 3.752, thus bringing the heavier varieties of the stone into group B, but in all cases where overlapping occurs, the colour, form, and the self-evident character of the stone are in themselves sufficient for cla.s.sification, the specific gravity proving genuineness. This is especially appreciated when it is remembered that so far science has been unable (except in very rare instances of no importance) to manufacture any stone of the same colour as the genuine and at the same time of the same specific gravity. Either the colour and characteristics suffer in obtaining the required weight or density, or if the colour and other properties of an artificial stone are made closely to resemble the real, then the specific gravity is so greatly different, either more or less, as at once to stamp the jewel as false. In the very few exceptions where chemically-made gems even approach the real in hardness, colour, specific gravity, &c., they cost so much to obtain and the difficulties of production are so great that they become mere chemical curiosities, far more costly than the real gems. Further, they are so much subject to chemical action, and are so susceptible to their surroundings, that their purity and stability cannot be maintained for long even if kept airtight; consequently these ultra-perfect "imitations" are of no commercial value whatever as jewels, even though they may successfully withstand two or three tests.

CHAPTER IX.

PHYSICAL PROPERTIES.

G--HEAT.

Another method of isolating certain stones is by the action of heat-rays. Remembering our lessons in physics we recall that just as light-rays may be refracted, absorbed, or reflected, according to the media through which they are caused to pa.s.s, so do heat-rays possess similar properties. Therefore, if heat-rays are projected through precious stones, or brought to bear on them in some other manner than by simple projection, they will be refracted, absorbed, or reflected by the stones in the same manner as if they were light-rays, and just as certain stones allow light to pa.s.s through their substance, whilst others are opaque, so do some stones offer no resistance to the pa.s.sage of heat-rays, but allow them free movement through the substance, whilst, in other cases, no pa.s.sage of heat is possible, the stones being as opaque to heat as to light. Indeed, the properties of light and heat are in many ways identical, though the test by heat must in all cases give place to that by light, which latter is by far of the greater importance in the judging and isolation of precious stones. It will readily be understood that in the spectrum the outer or extreme light-rays at each side are more or less bent or diverted, but those nearest the centre are comparatively straight, so that, as before remarked, these central rays are taken as being the standard of light-value. This divergence or refraction is greater in some stones than in others, and to it the diamond, as an example, owes its chief charm. In just such manner do certain stones refract, absorb, or reflect heat; thus amber, gypsum, and the like, are practically opaque to heat-rays, in contrast with those of the nature of fluorspar, rock-salt, &c., which are receptive. Heat pa.s.ses through these as easily as does light through a diamond, such stones being cla.s.sed as diathermal (to heat through). So that all diathermal stones are easily permeable by radiant heat, which pa.s.ses through them exactly as does light through transparent bodies.

Others, again, are both single and double refracting to heat-rays, and it is interesting to note the heat-penetrating value as compared with the refractive indexes of the stone. In the following table will be found the refractive indexes of a selection of single and double refractive stones, the figures for "Light" being taken from a standard list. The second column shows the refractive power of heat, applied to the actual stones, and consisting of a fine pencil blowpipe-flame, one line (the one twelfth part of an inch) in length in each case. This list must be taken as approximate, since in many instances the test has been made on one stone only, without possibility of obtaining an average; and as stones vary considerably, the figures may be raised or lowered slightly, or perhaps even changed in cla.s.s, because in some stones the least stain or impurity may cause the heat effects to be altered greatly in their character, and even to become singly or doubly refracting, opaque or transparent, to heat-rays, according to the nature of the impurity or to some slight change in the crystalline structure, and so on.

_Selection of Singly refracting stones._ _Indexes of Rays of_ LIGHT. HEAT.

Fluorspar 1.436 4.10 varies Opal 1.479 2.10 "

Spinel 1.726 1.00 Almandine 1.764 1.00 Diamond 2.431 6.11 double

_Selection of Doubly refracting stones._ _Indexes of Rays of_ LIGHT. HEAT.

Quartz 1.545 4.7 single and double Beryl 1.575 1.0 varies considerably Topaz 1.635 4.1 " "

Chrysoberyl 1.765 1.1 " "

Ruby 1.949 5.1 single and double

The tourmaline has a light-refractive index of 1.63, with a heat index of none, being to heat-rays completely opaque.

The refractive index of gypsum is 1.54, but heat none, being opaque.

The refractive index of amber is 1.51, but heat none, being opaque.

In some of the specimens the gypsum showed a heat-penetration index of 0.001, and amber of 0.056, but mostly not within the third point. In all cases the heat-penetration and refraction were shown by electric recorders. These figures are the average of those obtained from tests made in some cases on several stones of the same kind, and also on isolated specimens. Not only does the power of the stone to conduct heat vary in different stones of the same kind or variety, as already explained, but there is seen a remarkable difference in value, according to the spot on which the heat is applied, so that on one stone there is often seen a conductivity varying between 0.15 to 4.70.

This is owing to the differences of expansion due to the temporary disturbance of its crystalline structure, brought about by the applied heat. This will be evident when heat is applied on the axes of the crystal, on their faces, angles, lines of symmetry, etc., etc., each one of which gives different results, not only as to value in conductivity, but a result which varies in a curious degree, out of all proportion to the heat applied. In many cases a slight diminution in applied heat gives a greater conductivity, whilst in others a slight rise in the temperature of the heat destroys its conductivity altogether, and renders the stone quite opaque to heat-rays.

This anomaly is due entirely to the alteration of crystalline structure, which, in the one case, is so changed by the diminution in heat as to cause the crystals to be so placed that they become diathermal, or transparent to heat-rays; whilst, in the other instance, the crystals which so arrange themselves as to be diathermal are, by a slightly increased temperature, somewhat displaced, and reflect, or otherwise oppose the direct pa.s.sage of heat-rays, which, at the lower temperature, obtained free pa.s.sage.

Thus certain stones become both opaque and diathermal, and as the heat is caused to vary, so do they show the complete gamut between the two extremes of total opacity and complete transparency to heat-rays.