Many of the pegmatite dikes near Lake Buchanan in Llano and Burnet counties have produced some good red microcline specimens and cutting material ([Pl. III], A, and [fig. 16]). Many of these crystals are more pinkish than those in Gillespie County, but this is commonly due to the fact that the crystal faces of the Lake Buchanan area crystals are somewhat more weathered than the fresh Gillespie County crystals.
Numerous other local areas in the counties mentioned, as well as some localities in Hudspeth and Culberson counties, have also produced small amounts of red and pink microcline of gem quality.
Obsidian
Composition: volcanic glass. Structure: amorphous. Hardness: 5.0 to 5.5. Specific gravity: 2.3 to 2.5. Luster: vitreous. Color: black, dark gray, reddish, brown, bluish, and greenish. Streak: white. Cleavage: none. Fracture: conchoidal. Tenacity: brittle. Diaphaneity: translucent to nearly opaque. Refractive index: variable, about 1.45 to 1.53.
Gem-quality black and dark-gray obsidian has been found in Presidio County associated with extrusive igneous rocks. The obsidian in this area is too opaque to serve as attractive faceted stones but is found in pieces of sufficient size and quality to yield nice cabochons. Some of the small weathered pieces of this material resemble tektite in outward appearance; in fact, the “valverdites” mistaken originally for tektites are pebbles of weathered obsidian in terrace gravel of Val Verde County. Obsidian takes a high polish but is very sensitive to heat. Stones that are slightly overheated during grinding or sanding will quickly shatter.
Obsidian of gem quality has been reported also in Brewster County.
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.