Opal
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.