Orthoclase, an alkali feldspar, occurs in a range of colors, the most common being colorless. Adularia, a colorless, transparent variety from Adular-Bergstock in Switzerland, has a bluish white “schiller” or sheen, called adularescence.
Orthoclase feldspar occurs in intrusive igneous rocks and is one of the main constituents of granitic pegmatites. It is also found in metamorphic rocks such as schist and gneiss. Clear, colorless orthoclase occurs in Madagascar. Yellow and colorless cuttable material, cat’s-eyes, and some star stones occur in the gem gravels of Sri Lanka and Burma.
Feldspars are the most common rock-forming minerals at the Earth’s surface. They are divided into two groups: the alkali feldspars and the plagioclase feldspars. Orthoclase derives its name from the Greek for “break straight”, a reference to the stone’s perfect cleavage at near 90 degrees.