Labradorite
Composition: NaAlSi₃O₈, 50% to 30%; CaAl₂Si₂O₈, 50% to 70%. Crystal system: triclinic. Hardness: 6.0 to 6.5. Specific gravity: about 2.60. Luster: vitreous to sometimes pearly. Color: straw yellow, white, greenish, gray, reddish, bluish, and green. Sometimes shows a play of colors on particular cleavage surfaces. Streak: uncolored. Cleavage: three directions. Fracture: uneven to conchoidal. Tenacity: brittle. Diaphaneity: transparent to translucent. Refractive index: about 1.56. Dispersion: low.
Very fine facet-quality labradorite has been found about 20 miles south of Alpine, Brewster County. The labradorite occurs loose in the soil as slightly weathered or frosted cleavage fragments, commonly showing one or more crystal faces ([Pl. II], B). The pale-yellow or straw-yellow color of these fragments, as well as their lack of internal imperfections, makes these stones excellent gem material. Individual pieces that exceed three-fourths inch in their longest dimensions are rare. Cut stones of more than 5 or 6 carats from this locality are scarce. The source of this material is uncertain, but it is probably weathering out of an underlying igneous rock.