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.