Garnet is not one mineral but is the name given to a group of several
minerals that are very much alike. In fact, it often is impossible to
tell some of them apart without using special laboratory tests.
[Illustration: Garnet crystal forms include: A, trapezohedron; B,
dodecahedron; C and D, combination trapezohedron and dodecahedron.]
The garnet minerals have glassy to resinous lusters and are transparent
or translucent. A pocket knife will not scratch them, and some specimens
are too hard even for quartz to scratch. Two of the garnet minerals most
commonly found in Texas are _almandite_, an iron-aluminum silicate, and
_grossularite_, a calcium-aluminium silicate. Almandite has a deep-red
or a brownish-red color. Grossularite is pale green, brownish yellow,
cinnamon brown, or rose red.
Garnet minerals occur as crystals and as masses that are scattered
through some of the metamorphic and igneous rocks. After they have
weathered out of these rocks, the garnets make up a part of many sands
and sandstones. Because these minerals so commonly occur as crystals, it
is helpful to learn to recognize the crystal shapes.
Garnet minerals are found in the igneous and metamorphic rocks of both
central Texas and west Texas. In central Texas, they occur in ancient
Precambrian schist and pegmatite rocks of the Llano uplift area. Some of
these central Texas garnet localities are in northeastern Mason County,
central and northwestern Llano County, west-central Burnet County, and
northeastern Gillespie County.
In west Texas, garnets occur in metamorphic rocks in the Quitman
Mountains, which are southwest of Sierra Blanca in Hudspeth County, and
in the Mica Mine area, which is south of Van Horn near the
Hudspeth-Culberson County line. Garnets also have been found in igneous
rocks in the Franklin Mountains a few miles north of El Paso in El Paso
County.
Garnets that are found in metamorphic rocks such as schists were formed
when great forces squeezed and heated rocks far below the earth’s
surface. This heat and pressure caused elements in the rocks to join
together into different combinations to form new minerals, such as
garnets. Garnets that occur scattered through igneous rocks, such as
some pegmatites and granites, cooled and crystallized from hot, igneous
fluids when the rocks themselves formed.
Most Texas garnets are not transparent. A few, however, are clear enough
to be used as gemstones. These can be cut, polished, and mounted in
rings, brooches, bracelets, and earrings. Although some garnet is widely
used as an abrasive, none from Texas has been produced for this purpose.