Spinel
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.
Fig. 17. Common crystal form of spinel.