Garnet
The garnet group of minerals is variable in composition. Listed below are the pure members of this group, but garnets found in nature are usually a mixture of two or more of these end members.
Aluminum garnet— Grossularite (calcium-aluminum garnet), Ca₃Al₂(SiO₄)₃ Pyrope (magnesium-aluminum garnet), Mg₃Al₂(SiO₄)₃ Almandite (iron-aluminum garnet), Fe₃Al₂(SiO₄)₃ Spessartite (manganese-aluminum garnet), Mn₃Al₂(SiO₄)₃ Iron garnet— Andradite (calcium-iron garnet), Ca₃Fe₂(SiO₄)₃; may contain magnesium, titanium, and yttrium Chromium garnet— Uvarovite (calcium-chromium garnet), Ca₃Cr₂(SiO₄)₃
Since almandite is the only variety of garnet known to occur commonly in gem quality in Texas, the following properties are for almandite except where noted.
Crystal system: isometric (all varieties). Hardness: about 7.5. Specific gravity: 4.25. Luster: vitreous to resinous. Color: red, deep red, and brownish red (other varieties also yellow, white, orange, pink, black, and green). Streak: white. Cleavage: none. Fracture: subconchoidal to uneven. Tenacity: brittle to tough. Diaphaneity: transparent to subtranslucent. Refractive index: about 1.83.
Good crystals of gem-quality almandite garnet have been found in Llano, Blanco, Burnet, and Gillespie counties. In southeast Llano County, northwest Blanco County, and northeast Gillespie County, the stones mostly occur in stream gravels where they have collected after being weathered out of compact mica schists. Owing to the fact that most of the garnets have not been transported very far from their source, the stones commonly show good crystal form ([Pl. II], A). All of the garnets from one locality commonly do not have exactly the same crystal form. The garnets are mostly widely scattered in the stream gravels but can be found concentrated behind rocks and on small gravel bars.
Many of the crystals are less than one-eighth inch in diameter; however, good crystals one-fourth to one-half inch in diameter are common. Most of the stones are too fractured or have too many inclusions to yield gems, but many transparent stones have been found. The transparent crystals usually yield flawless deep red faceted stones of 2 carats or less. Some of the stones that contain too many inclusions to facet are cut as cabochons and are then often known as carbuncle.
Small garnet fragments have been found in streams and in gneisses and pegmatites near Castell, Llano County, but they are not commonly of gem quality.
Occasional small gem-quality garnets have been found in pegmatites and contact metamorphic zones in Burnet County. Garnets have also been found in several other counties, notably Mason, El Paso, Hudspeth, and Culberson, but no stones of facet quality have been reported.