Sanidine
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.