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ColorlessTopaz
As a mineral, this is far and away the most
common variety. of transparent topaz, although it is of limited
importance as gemstone. All colorless gemstones compare most
unfavorably with diamond, their lower refractive indices an
lesser dispersion making them far less attractive. Topaz has
the advantage over the others of being found in large stones
and great quantities, and of having good hardness and luster.
Therefore it is of some interest to hobbyists.
Appearance Gemstones are generally
free, or almost sc of inclusions because they can be culled
from the large quantities available; but they may show signs
of extensive internal cleavage. In large stones (of a few
centimeters) the mineral's weak birefringence can be detected
with the a of a lens, the facet edges appearing double when
see through a flat facet of the stone. The stones easily acquire
good luster and are only disappointing on the rare occasions
when they are given the brilliant cut, prompting comparison
with diamonds, which have a very different lust, and fire.
Distinctive features Colorless topaz differs
little from the colorless varieties of other minor gems, which,
with the exception of quartz, are all much rarer. Incipient
cleavage where present, may be a distinguishing feature. If
the edges appear double, denoting birefringence, this can
distinguish it from synthetic colorless spinel, which has
a sir large density; but it cannot differentiate it from other
min gemstones such as colorless beryl, tourmaline, and corundum.
To distinguish it with any certainty from these, the refractive
indices must be determined.
Occurrence It is found in various parts of
Brazil, the Soviet Union, the United States, Germany (Saxony),
a Japan. Africa, too, has a plentiful supply of colorless
topaz in the tin-mining areas of both Nigeria and Zaire. It
is also found in Namibia.
Value One of the lowest for
transparent gemstones, even though it is greatly appreciated
by amateur cutters as a raw material. Apparently, it is being
used increasingly for the production of blue topaz by means
of irradiation.
Simulants and synthetics It is neither imitated
nor produced synthetically
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