The name "moonstone" is reputed to have originated in India, because of the bluish billowy sheen resembling moonlight on the water in that country. It was once thought that moonstones were washed up on shore after a full moon and high tide, and some say this may be where the phrase "once in a blue moon" originated.Popular with the Romans, who thought it was formed out of moonlight, and in India, where it is considered a sacred zodiac gem; moonstone is one of the most coveted varieties of feldspar. Other names for moonstone include adularia (a variety found in the European Alps near the Adula Group) and selenite (from the Greek selene, meaning – moon.
Moonstone is a potassium-rich orthoclase member of the feldspar group of minerals and is closely related to sunstone and labradorite. The name feldspar comes from the German feldt spat, meaning “field stone.” This is because when feldspar weathers, it releases large amounts of plant nutrients, such as potassium, which enrich the soil. Moonstone shows a blue-whitish opalescence called adularescence (sometimes described as a billowy light or shimmer) that glides over the surface of the gem. Interference phenomena, due to the intergrowth of two different types of feldspar with different refractive indexes, from the gem’s layered structure are the cause of this effect. Moonstones are often cut as cabochons to maximize this effect.
Moonstones are semi precious stones and are usually pearly white in color. There are Moonstones that are used in moonstone jewelry that range in colors from colorless to brown, yellow, pink, green, and gray. The clarity of Moonstones especially those used in Moonstone jewelry such as earrings have a clarity that ranges from transparent to translucent to transparent. Moonstones that are of a higher quality have a blue sheen with a colorless body and perfect clarity.
Fine moonstone is quite rare and becoming rarer. It is mined in Sri Lanka and Southern India. The rainbow variety can be found in India and Madagascar.
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