TOPAZ, a mineral, ranked by mineralogists among gems (q. v.), and the finer. varieties of which are much valued both for their luster and the beauty of their colors. It is composed chiefly of alumina and silica, the former, in general, more than 50 per cent of the whole, with fluoric acid, and usually a little oxide of iron. It is found generally in primitive rocks, and in many parts of the world. A. crystal 19 ounces in weight was found in the Cairngorm mountains in Aberdeenshire, Scotland; and fine topazes are sometimes found in that part of Scotland, in Cornwall, and in the Mourne mountains in Ireland. Fine topazes are found in Ceylon, but those most prized by jewelers are generally from Brazil. The finer varieties of topaz are in general found either crystallized, or as small rolled masses, which may have been formed from crystals, in alluvial soil. Topaz is either colorless, or red, blue, green, or yellow, in great variety of shades. Its cystals are rhombic prisms, generally terminated by four-sided pyramids,
but often variously bevelled and acuminated. The prisms are finely striated. The cleavage parallel to the base of the prism is easy. The specific gravity is about 3.5. The luster is vitreous. Topaz is translucent or almost transparent on the edges. It is harder than quartz. It is rendered very electric by heat or friction, and by this property a topaz may at once be distinguished from a diamond or ruby, for which otherwise, when cut and set, it might readily be mistaken. A coarse variety of topaz, called pyrophysalite, occurs near Fabian, in Sweden, which is not crystallized. It is greenish white. When reduced to powder, it can be used as emery for grinding and polishing. —Topaz derives its name from the topazion of the ancients, which, however, seems to have been a totally different mineral.