_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.