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 shimmer 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.