_Composition_: MgAl₂O₄ (magnesium may be replaced in part by ferrous
iron or manganese and the aluminum by ferric iron and chromium).
_Crystal system_: isometric. _Hardness_: 8. _Specific gravity_: 3.5 to
4.1. _Luster_: vitreous to sub-metallic. _Color_: black, pink, red,
blue, green, yellow, brown, and violet. _Streak_: white. _Cleavage_:
one direction, imperfect. _Fracture_: conchoidal. _Tenacity_: brittle.
_Diaphaneity_: transparent to opaque. _Refractive index_: variable,
approximately 1.72 to 2.00.
In many areas of the world, fine quality, beautifully colored,
transparent spinels are found and used as gems. The only gem-quality
spinel reported thus far in Texas is black and opaque. Near Eagle Flat
in Hudspeth County, black spinel crystals have been found associated
with augite and natural glass; these minerals are weathering out of an
intrusive igneous rock. The spinel crystals have an octahedral form
which is common for this mineral (fig. 17). Most of the spinels are free
of flaws, but because of their black color they have little value as
gems. The crystals are found loose in the sand of streams near the
outcrops of the igneous rock or embedded in the rock. They seldom exceed
half an inch in diameter. These stones are primarily sought by
collectors.
[Illustration: Fig. 17. Common crystal form of spinel.]