[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.
[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.