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 This
article is about the mineral or gemstone, for other
uses see: Topaz (disambiguation).
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. It can also be irradiated,
turning the stone a light and distinctive shade of
blue.
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, Czech Republic, Saxony, Norway,
Sweden, Japan, Brazil, Mexico, and the United States.
The name "topaz" is derived from the Greek topazos,
"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.
Topaz is also the birthstone of November. |
