| Moonstone is the name given to the member of feldspar group that displays a sheen effect (adularescence or schiller). The 2 main categories of moonstones are the traditional "true moonstone" composed of orthoclase and albite and the "rainbow moonstone" which is, in fact, labradorite feldspar and less valuable than the orthoclase variety.
Moonstone body color can range from colorless to white, gray, brown, yellow, orange, green, or pink. But its beauty is in its sheen, which can be white to deep blue and create a sweet color that seems to float across the convex surface of the stone when it is cut en cabochon. This effect is especially beautiful when the stone is displayed on a dark background such as an evening dress. The sheen effect is caused by the intergrowth of two different types of feldspar with different refractive indexes: These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus a separating and layering of these two phases when the stone cools down during its genesis.
The resulting shiller effect is caused by a ray of light entering a layer and being refracted back and forth by deeper layers before leaving the crystal. This refracted ray has a different character than the incident one and this produces the moonlike glow. If feldspar is very common on earth, fine moonstone is quite rare and is becoming rarer nowadays. Feldspar is the group name applied to several crystalline aluminosilicate minerals that are found in abundance in the earth's crust in many localities throughout the world, especially in pegmatites. Mogok is now the typical origin for the finest moonstones as the traditional mines in Sri Lanka are now closed.
Finest classical quality is known to show both a fine blue sheen and a colorless body color but recently an extremely rare and spectacular "Golden moonstone" from Mogok, Burma (Myanmar) has arrived in the market, and the very few stones produced are sold at very high prices. Moonstone is considered a sacred stone in India, often displayed on a yellow cloth, also considered a sacred color.
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