Semi-Precious Gemstones: Moonstone
Rainbow Moonstone (Selenite) used in Jewelry
Source: Australia, Brasil, India, Madagascar, Myanmar, Russia, Sri Lanka, Tanzania, USA
Birthstone: June (Alternate: Alexandrite)
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 is a translucent to opaque tectosilicate which is a member of the feldspar group (orthoclase feldspar). Moonstone is found as a constituent rock-forming component in feldspar-rich granitic and syenitic pegmatites. Moonstone, sunstone, and labradorite are all comprised of feldspars.
Moonstone (aka "rainbow moonstone") is composed of adularia (alkali feldspar) which is a form of orthoclase (Greek for "straight fracture") that typically crystallizes at lower temperatures due to hydrothermal activity; or plagioclase feldspar oligoclase (plagioclase is Greek for "oblique fracture") which is a variety of albite, sheet mica, and orthoclase - a gemstone-quality silicate of potassium and aluminium.
Titanium-Backed Moonstone (photo: Dan Dennis)
Crystal twinning is a common occurrence in most plagioclases. Basal cleavage forms right angled prisms with good cleavage in one of the two directions, and perfect cleavage in the other.
Moonstone's Adularescence (Schiller Effect)
Moonstone's characteristic blue/white scheen is caused by an optical phenomenon that is referred to as the "schiller effect," "aventurescence" (aka "aventurization"), or "adularescence." The "play of light" created by adularescence is caused by thin, alternating layers of feldspar orthoclase and albite. These layers all interact to scatter reflected light when incident light rays refract off of lamellas (lamellar intergrowths) inside the stone.
Moonstone Crystallography, Chemistry, Physical Properties
Moonstone Optical Properties
Sometimes as titanium backing is added to the stone (known as "titanium-backed moonstone") to enhance or create a prismatic effect when light is reflected back through the moonstone. The optical qualities of Moonstone are sometimes mischaracterized as "opalescent." Some varieties of Moonstone also display chatoyancy or a multi-rayed star effect.
Moonstone is typically cut en cabochon due to its softness and opacity, and is a soft gemstone that is easily scratched. The most desirable effects of adularescence are achieved when the stone is cut on its optimal crystal axis.
Bibliography and Reference on Moonstone
1. Paul R. Shaffer, Rocks, Gems and Minerals . Martin's Press
3. Renee Newman, Gemstone Buying Guide . International Jewelry Publications; 2nd edition
3. Antoinette L . Matlins, Antonio C. Bonanno, Gem Identification Made Easy . Gemstone Press