Leisure hours among the gems

by Augustus C. Hamlin

To estimate the value of a rough diamond and ascertain its purity is often a difficult task, and one that requires both education and tact. For the surface of the natural gem, especially the modified and spheroidal crystals, is generally dull and chatoyant. This singular appearance, which has been erroneously represented as a thin crust, is in reality produced by the salient edges of the laminæ of which the stone is composed. The same or a similar effect may be artificially produced in the polished diamond by heating it to a white heat, as has been seen in the diamonds injured in the great fires of Hamburg and Chicago.

Barbot, the French jeweller, declared that he had discovered a means by which the apparent rough, translucent coating could be changed into perfect transparency so as to give a clear view of the condition of the interior. This statement, however, is very much doubted by lapidaries.

When the rough diamond is placed in turpentine, alcohol, or even water, it appears more transparent than before and like a bubble of air. We have therefore sometimes thought that Barbot had really discovered some fluid of high refractive power, immersed in which the rough diamond became transparent. For it is a well-established fact known among chemists, that rough gems of a lower refractive power, like sapphire, chrysoberyl, spinel, etc., if placed in a fluid possessing an equal refractive energy, like that of muriate of antimony, become clear, and the observer is enabled to look through them. This discovery strengthens Barbot’s statement and places it within the possibilities; but as yet we know of no fluid that possesses a refractive power equal to the diamond. Topaz, emerald, and other gems of low refractive power can be readily examined by using the oil of cassia, which has the refractive energy of 1.64.

The rough diamonds often exhibit various colors, but generally of faint tints. Sometimes the hue is not perfectly distributed throughout the stone, but may be confined to a part of it, as in the sapphire, or even in one of its laminæ. It is a singular fact that the external coverings of the mineral often contain the shades which render the rough gem of a disagreeable hue, and which may be made to disappear by the process of cutting. Therefore, it sometimes requires experience and tact to determine the value of a rough diamond which possesses a decided hue. However, those stones which exhibit a greenish or reddish color are considered safe stones for investment, and will develop into superior gems after cutting. The bluish and the blackish are thought to be harder than the others; while the yellowish and grayish colors indicate inferior stones. The brownish and yellowish hues may disappear in process of cutting, but there is greater uncertainty in these shades than in the others. Sometimes a stone which promises to yield a perfectly white gem, after cutting turns out to be of a decided disagreeable tinge. Therefore the cutting of a rough diamond is often attended with some hazard.

The Hindoos have a practice of examining their stones before purchase by placing them in an aperture in the wall, with a lighted lamp in the rear. And to ascertain their color they take them under the cover of a tree thick with foliage, where the verdure of the shade quickly reveals any other tinge, especially the bluish. It is quite difficult to distinguish the snow-white, except by contrast. The officers of the Junta Diamontina, in Brazil, have a rough way of testing the native diamonds from quartz, white topaz, and spinel, by rubbing them together close to the ear. The sharp tone which is thus produced is characteristic, but the test requires a keen and practised ear.

Specific gravity is one of the best tests for determining the nature of polished stones. By this means some of the precious minerals can be detected with ease, like the zircon and the emerald, notwithstanding their color. But when applied to the diamond the experimenter should remember that its weight is exactly that of the white topaz; yet to distinguish them, the degree of hardness or the phenomena of electricity will at once enable the expert to discriminate with ease and without a doubt. For the best methods of ascertaining the specific gravity of the stones, we must refer the reader to elementary works on chemistry and physics.

The test of its electrical phenomena is an important one, as one can thereby almost instantly detect it from other gems, especially the white topaz. When rubbed it exhibits vitreous electricity like glass, but loses it in a very few moments.

Another curious phenomenon, called single refraction, enables the observer to distinguish the mineral from all other gems except the garnet and spinel, the others having double refraction, or, in other words, giving a double image of a candle-light when it is viewed through their facets.

For the purpose of observing this phenomenon, Sir David Brewster invented an instrument which he called a lithoscope. It consisted of a small glass prism which moved around a fixed joint so that the lower surface of it could be laid upon the surface, or a facet of the stone to be examined. In this position, the two surfaces being parallel, the image reflected from the lower surface of the prism would coincide with that reflected from the surface of the stone. A drop of the oil of cassia or of sulphuret of carbon is placed between the prism and the facet, and then the observer turns a screw to raise the prism a little round its joint. The effect of this is to separate the image of a light or a small luminous aperture as given by the prism from that given by the facet; and the difference in the intensity and the color of these two images is an infallible indication of the nature of the stone. The image from the diamond will be many times brighter than that reflected from the face of the prism when testing any of the other precious stones.

A simpler mode is sometimes adopted by experts, but it requires some dexterity to exhibit the property. The method is this: the diamond is held up to the eye, and a needle point or a small hole pierced in a card is looked at. If the object is seen double, as if there were two needle points or two holes, then the stone examined is not a diamond, as but one aperture should be seen.