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Topaz (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. |
