topaz: A variably coloured aluminiumsilicate mineral,Al2(SiO4)(OH,F)2, that forms orthorhombiccrystals. It occurs chieflyin acid igneous rocks, such as granitesand pegmatites. Topaz is valuedas a gemstone because of its transparency,variety of colours (the wineyellowvariety being most highly prized), and great hardness (8 on theMohs’ scale). When heated, yellow orbrownish topaz often becomes arose-pink colour. The main sources oftopaz are Brazil, Russia, and the USA.
