Augite
Composition: CaMgSi₂O₄; may also contain iron, aluminum, and sometimes titanium. Crystal system: monoclinic. Hardness: 5 to 6. Specific gravity: 3.2 to 3.6. Luster: vitreous to dull. Color: dark greenish brown and greenish black. Streak: light grayish green. Cleavage: two directions, poor. Fracture: conchoidal to uneven. Tenacity: brittle. Diaphaneity: opaque to translucent. Refractive index: variable, about 1.60 to 1.71.
Augite of gem quality occurs near Eagle Flat, Hudspeth County, Texas. Although this material is very dark greenish brown and not commonly thought of as a gemstone, lapidaries have used it to fashion black faceted stones and cabochons that resemble obsidian. Most of the augite occurs as loose pieces and crystal fragments that have weathered out of nearby igneous rocks; the augite can also be found in situ in the igneous rocks.
Specimens and pieces of cutting quality 1 inch in diameter are common, and fragments over 2 inches in diameter have been found. The augite is associated with black spinel and some dark gray to black pieces of natural glass. Although the faceted and cabochon-cut stones are not particularly attractive, some of the larger pieces of augite might be utilized for carving.