_Composition_: KAlSi₃O₈; commonly contains some sodium. _Crystal
system_: monoclinic. _Hardness_: 6. _Specific gravity_: 2.57 to 2.58.
_Luster_: vitreous to pearly. _Color_: colorless, white, pale yellow,
and gray. _Streak_: uncolored. _Cleavage_: three directions.
_Fracture_: conchoidal to uneven. _Tenacity_: brittle. _Diaphaneity_:
transparent to subtranslucent. _Refractive index_: 1.52 to 1.53.
Some feldspars, including sanidine, show a nice blue sheen in reflected
light parallel to certain crystallographic directions. Stones having
this property are called moonstone. A clear yellowish sanidine showing
an attractive blue sheen has been found in Brewster, Jeff Davis, and
Presidio counties. The individual pieces are small, the average size
being about one-eighth inch. The sanidine is found loose in the soil at
some localities where it has weathered out of rhyolite, and specimens of
the sanidine in the parent rock are not difficult to obtain. Very small
cabochons can be cut from this material, but few lapidaries have done so
because inexpensive larger pieces of moonstone can be obtained easily
from foreign sources. However, the west Texas sanidine does show a blue
sheen when cut and polished.