Topaz

Topaz, a mineral, ranked by mineralogists amongst Gems (q.v.), and the finer varieties of which are much valued both for their lustre and the beauty of their colours. It is composed chiefly of alumina and silica, the former, in general, more than 30 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—Russia, Siberia, Saxony, Australia, the United States, Cornwall, Aberdeenshire, the Mourne Mountains in Ireland—but those most prized by jewellers are generally from Brazil. Topaz may be colourless, light blue or green, rose-pink, orange, or straw yellow, in great variety of shades. Its crystals are rhombic prisms, generally terminated by four-sided pyramids, but often variously bevelled and acuminated. The prisms are finely striated. The specific gravity is about 3.5. Topaz is translucent or almost transparent on the edges, and 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 Fahlun in Sweden, which is not crystallised; when reduced to powder it can be used as emery for grinding and polishing. The so-called 'oriental topaz' is a yellow corundum; the 'false topaz' is a yellow variety of vitreous quartz. Topaz derives its name from the Topazion of the ancients, which, however, seems to have been a totally different mineral, probably chrysolite.