Topaz

The mineral topaz is a silicate of aluminium and fluorine with the chemical formula (AlF)₂SiO₄. It is orthorhombic[?] and its crystals are mostly prismatic terminated by pyramidal and other faces, the basal pinacoid often being present. It has an easy and perfect basal cleavage and so gemstones or other fine specimens should be handled with care to avoid developing cleavage flaws. The fracture is conchoidal[?] to uneven. Topaz has a hardness of 8, a specific gravity of 3.4-3.6, and a vitreous lustre. Pure topaz is transparent but is usually tinted by impurities; typical topaz is wine or straw-yellow. They may also be white, gray, green, blue, or reddish-yellow and transparent or translucent. When heated, yellow topaz often becomes reddish-pink.

Topaz is found associated with the more acid rocks of the granite and rhyolite type and may be found with fluorite and cassiterite[?]. It can be found in the Ural and Ilmen[?] mountains, Czechoslovakia, Saxony, Norway, Sweden, Japan, Brazil, Mexico, and the United States.

The name "topaz" is derived from the Greek "to seek," which was the name of an island in the Red Sea that was difficult to find and from which a yellow stone (now believed to be a yellowish olivine) was mined in ancient times. In the Middle Ages the name topaz was used to refer to any yellow gemstone, but now the name is only properly applied to the silicate described above.

See also: List of minerals