topaz, aluminum silicate mineral with either hydroxyl radicals or fluorine, Al₂SiO₄(F,OH)₂, used as a gem. It is commonly colorless or some shade of pale yellow to wine-yellow; pale blue and pale green also occur, but natural red stones are uncommon. Some natural yellow stones lose their yellow coloring when heated and become permanently pink ("pinked" topaz). The stone is transparent with a vitreous luster. It has perfect cleavage on the basal pinacoid, but it is nevertheless hard and durable. The brilliant cut is commonly used. Topaz crystals, which are of the orthorhombic system, occur in highly acid igneous rocks, e.g., granites and rhyolites, and in metamorphic rocks, e.g., gneisses and schists. Important sources of topaz are in Russia, Siberia, Brazil, Australia, and Mexico and in New Hampshire, Colorado, and Utah in the United States. The name topaz is commonly but incorrectly used for various other yellow stones, e.g., for citrine quartz.

Aluminum silicate mineral, Al₂SiO₄(F,OH)₂, that is valued as a gemstone. It is formed by fluorine-bearing vapours given off during the last stages of the crystallization of igneous rocks. Pure topaz may be colourless and, when brilliant-cut, has been mistaken for diamond. It may also be various shades of yellow, blue, or brown. Imperial topaz, with vivid reddish orange colour, from Minas Gerais, Braz., is very highly valued.

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