_Composition_: SiO₂·nH₂O. _Structure_: amorphous. _Hardness_: 5.5 to
6.5. _Specific gravity_: 1.9 to 2.3. _Luster_: subvitreous to pearly.
_Color_: white, bluish, pink, brown, yellow, and gray. _Streak_:
white. _Cleavage_: none. _Fracture_: conchoidal. _Tenacity_: brittle.
_Diaphaneity_: transparent to nearly opaque. _Refractive index_: 1.43.
Opal other than as fossil or opalized wood (pp. 20-21) occurs at the
following several localities in Texas.
Approximately 16 miles south of Alpine, Brewster County, precious opal
occurs in very small seams and as cavity fillings in very hard
pinkish-brown rhyolite. This opal is milky or bluish and commonly
exhibits small flashes of blue, green, red, and orange fire. Individual
pieces of this opal are mostly quite small, rarely over one-fourth inch
in diameter, and very difficult to remove from the tough rhyolite
matrix. Local lapidaries have cut interesting cabochons from this
material in which several small patches of opal that are close together
in the matrix are included in the same cabochon.
Small finds of opal associated with rhyolites and basalts have come from
other localities in west Texas, but the opal mostly does not display
enough play of colors to warrant its use as gem material.
Near Freer, Duval County, some very attractive common opal has been
found. The opal is colored various shades of pink, blue, and yellow and
in certain local areas occurs as fragments that are cemented together by
clear chalcedony. Various colors are commonly found in the same piece,
and such material yields handsome cabochons. Although the area has never
been worked commercially, it has been hunted by collectors and cutters
for several years.