The name "moonstone" (Pierre de lune French, Mondstein German, Piedra de luna Spanish or Italian) is derived from the Greek name "Selenite" or goddess of the moon. Moonstone is also known as "adularia" which derived its name from Adula, the location where moonstone is found in the Italian/Swiss Alps, or "selenite", from the Greek word selene or "moon". The principle sources for the highest quality moonstone are India, Sri Lanka (Ceylon) and Tanzania. Moonstone's characteristic blue/white scheen is reffered to as called "schiller" (moonshine). Moonstone is a birthstone of June.
Moonstone is found as a constituent rock-forming component in feldspar-rich granitic and syenitic pegmatites. Sunstone, moonstone, and labradorite are all comprised of feldspars. Moonstone is a transparent to opaque oligoclase, which is a variety of plagioclase albite, sheet mica, and orthoclase (straight fracture) alkali feldspar called "adularia" which is a gemstone quality silicate of potassium and aluminium.
The opalescent quality of moonstone is caused by a thin layers of feldspar, and is referred to as "adularescence". Adularescence (aka play of light) is a reflective phenomenon caused by incident light rays refracting off of scales and/or lamellas or lamellar intergrowths inside the stone, scattering the reflected light. Some varieties of Moonstone also display chatoyancy or a multi-rayed star effect.
Moonstone is easily scratched, with a mineral hardness of 6 on the Mohs scale. Moonstone is in the triclinic crystal system, and has a blocky or tabular crystal habit. Crystal twinning is a common occurrence in most plagioclases.
Moonstone has a refractive index of 1.533 to 1.552 and a vitreous to pearly surface luster. Basal cleavage forms right angled prisms with good cleavage in one the two directions and perfect cleavage in the other. Moonstone is typically cut in a cabochon due to its softness and opacity. The most desirable effects of adularescence are achieved when the stone is cut on its optimal crystal axis.
Chemical composition: (Feldspar) NaAlSi3 O8, (Albite) CaAl2 Si2 O8
Semi-Precious Stones