_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.