This carbonate is principally used to point rock-drills and for other engineering purposes. The coarse variety of crystallized diamonds is called bort, and as this is unfitted for gem purposes because of imperfections, it is ground into powder and used for cutting and drilling precious stones. White sapphires, white zircons, white topaz, and rock-crystal sometimes pass for diamonds. The first two are heavier, the topaz lacks brilliancy, and the crystal is lighter than the diamond. It is also the case that these four stones, especially the crystal, are easily scratched by a diamond. The best style of cutting for a diamond is the brilliant, of 66 facets, including the table and culet. The proper proportions of a well cut brilliant is ⅓ for the crown and ⅔ for the culet. The table and culet must also be in proportion to the size of the stone. CORUNDUM. This many-colored mineral, composed of nearly pure alumina, produces gems which in some cases are more valuable even than diamonds. The ruby, sapphire, Oriental emerald, Oriental topaz, Oriental amethyst, Oriental aquamarine, Oriental chrysolite, Oriental hyacinth, star ruby, star sapphire, star topaz, and ruby and sapphire cat’s-eyes are all corundums of different colors. The ruby is a red sapphire, and the Oriental topaz a yellow sapphire, while the Oriental emerald is a green sapphire, etc., etc. In hardness corundum ranks next to the diamond, ranking No. 9 in Moh’s scale. The specific gravity is 3.9 to 4.1, the crystallization rhombohedral, and cleavage basal, the crystals breaking across the prism with nearly a flat surface. In lustre, the corundum is vitreous, its refraction double but not to a high degree, and it is susceptible of electricity by friction, which the polished specimens especially retain for a considerable time. Corundum is unaffected by chemicals, and is infusible alone, but in combination with a flux it melts with difficulty into a clear glass. The chemical composition of precious corundum is: Alumina 98.5 Oxide of iron 1.0 Lime 0.5 ───── 100. Thus it will be seen that corundum is composed almost wholly of alumina,—one of the constituents of common clay, which, when colored by traces of metallic oxides, chrome, etc., produces a greater variety of precious stones of a high rank than any other mineral. THE RUBY. The red sapphire or ruby is the most valuable of the corundum family, and when found of a good color, pure and brilliant, and in sizes of one carat and larger, it is much more valuable than a fine diamond of the same size. Fine rubies larger than 1½ to 2 carats are very rare, and when a fine stone from 3 to 5 carats is offered for sale, the price mounts into the thousands.