_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.