From the Greek for "straight fracture", in allusion to its perpendicular cleavages. Orthoclase is feldspar (at the potassium-bearing end of the alkalai feldspars, for us mineral geeks). Crystals frequently twin. It is the main constituent of China clay. Most moonstone is an opalescent variety of orthoclase, which displays a schiller, or shenn. In this context I am referring to the common, dense opaque material. Moonstone is under a separate heading.
Chemical Formula: KAlSi3O8
Birthstone for Cancer