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.