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.
Beryl
Composition: Be₃Al₂(SiO)₆. Crystal system: hexagonal. Hardness: 7.5 to 8.0. Specific gravity: 2.63 to 2.80. Luster: vitreous. Color: pale blue, blue, green, yellow, brownish, pink, and colorless. Streak: white. Cleavage: one direction, very imperfect. Fracture: conchoidal to uneven. Tenacity: brittle. Diaphaneity: transparent to subtranslucent. Refractive index: 1.56 to 1.60. Dispersion: low.
Gem-quality beryl has not been reported in Texas. A discussion of beryl is included herein because the writer believes it likely that beryl of gem quality will be found in Texas as a result of future investigations and exploration.
Beryl crystals have been found in pegmatite dikes in Llano, Blanco, and Gillespie counties. These crystals are commonly several inches long and exceed 1 inch in diameter but are very badly fractured. Most of the beryl crystals do not approach gem quality and are entirely unsuitable for any lapidary use. The color of the crystals found thus far is bluish, greenish, pinkish brown, yellowish, and colorless. Some very tiny colorless beryl crystals have been found that are transparent, but thus far such crystals have been too small to be cut into gems.
Fine blue beryl crystals have been found in the Franklin Mountains near El Paso, Texas. Unfortunately, these crystals are so badly flawed and fractured that they are not suitable for lapidary use.
It seems likely that careful prospecting of Texas pegmatites will reveal at least some gem-quality beryl.