Topaz, an aluminum fluorosilicate, is a mineral especially prized by
collectors because many specimens are gemstones. Topaz is transparent,
has a glassy luster, and is quite hard (neither quartz nor a steel file
will scratch it). The topaz that has been found in Texas is either
colorless, pale blue, or sky blue. This mineral is fairly heavy—its
specific gravity is 3.4 to 3.6. It cleaves perfectly in one direction
(called basal cleavage), and some of the cleavage fragments have a flat,
slabby appearance.
Topaz is commonly found as prism-shaped crystals, as cleavage fragments,
and as irregular grains. Some fragments of topaz look like quartz.
Topaz, however, is harder and heavier than quartz, and it has perfect
basal cleavage, which quartz does not have.
In Texas, crystals, grains, and cleavage fragments of topaz occur in the
Llano uplift area of central Texas. They are found near Streeter and
Grit in west-central Mason County and near Katemcy in northern Mason
County. Here, some of the topaz occurs in Precambrian pegmatite veins
that cut through granite rocks. Most of the topaz, however, is found as
pebbles in the gravels of nearby creeks, where it has washed after
weathering out of the rocks.
[Illustration: Topaz crystal from near Streeter, Mason County, Texas.]
Topaz probably originates when hot fluids move up out of molten magma
into cracks and cavities in the surrounding rocks. There, the fluids
react with elements in the rocks to form the topaz.
Topaz is a good gemstone because, in addition to its beauty, it is hard
and is not easily marred by scratches. The Mason County topaz makes
excellent gemstones. Most of it is beautiful and clear and is either
colorless or of a pleasing blue color. These stones are cut, polished,
and mounted in rings and other jewelry. A number of specimens of this
Mason County topaz are displayed in museums.