Topaz, a precious stone, a silico-fluoride of alumina, consisting, in 100 parts, of alumina 48 to 58, silica 34 to 39, and fluorine 15 to 18'5. Its specific gravity is 3.4 to 3.65; its hardness is 8, or between that of quartz and sapphire. It is usually colorless, but is sometimes blue, green, or red. The yellow Brazilian topaz when heated becomes reddish, while the Saxon wine-colored topaz loses its color entirely. Topaz is pyro-electric; it crystallizes in the trimetric or rhombic system, the prism generally having dissimilar extremities. When heated in the blowpipe flame it becomes covered with small blisters, while a coarse variety called physalite (Gr. фύσειν, to blow) swells up when heated. Its principal localities are : the Ural and Altai mountains, Kamtchatka, Villa Rica in Brazil (of a deep yellow color), Alten-berg in Saxony, and the Mourne mountains in Ireland; in the United States, at Trumbull and Middletown, Conn., and at Crowder's mountain, N. C. Physalite is found in Norway and Sweden in very large crystals; one weighed 80 lbs. The topaz is not very highly valued as a gem, though fine specimens sometimes bring very good prices. Tavernier speaks of one belonging to the Great Mogul weighing 157 carats, which was valued at 181,000 rupees.

The principal supply is from Brazil, which furnishes about 40 lbs. annually. The white and rose-red are the most valuable. The former are called by the Portuguese pingas d'agoa (drops of water), and when cut resemble the diamond in brilliancy. The oriental topaz is the yellow variety of transparent corundum, and belongs to the family of sapphires. (See SAPPHIRE.) A yellow variety of quartz is sometimes called false topaz.