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Definition: topaz from Philip's Encyclopedia

Transparent, glassy mineral, aluminium fluosilicate, Al2SiO4(F,OH)2, found in pegmatites. Its crystals are orthorhombic system columnar prisms. Topaz is colourless, white, blue or yellow; some large crystals are of gem quality. Hardness 8; r.d. 3.5.

Summary Article: topaz
From The Columbia Encyclopedia

(tō'păz), 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.

The Columbia Encyclopedia, © Columbia University Press 2018

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