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.