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Moonstone is typically a potassium aluminium silicate, with the chemical formula KAlSi3O8.
The most common moonstone is of the mineral adularia. The plagioclase feldspar oligoclase also produces moonstone specimens. Its name is derived from a visual effect, or sheen, caused by light reflecting internally in the moonstone from layer inclusion of different feldspars.
Moonstone's delicate beauty and its long heritage make it perhaps the most familiar gem quality member of the feldspar group. Moonstone is composed of two feldspar species, orthoclase and albite. The two species are intermingled. Then, as the newly formed mineral cools, the intergrowth of orthoclase and albite separates into stacked, alternating layers. When light falls between these thin, flat layers, it scatters in many directions producing the phenomenon called adularescence.
Deposits of moonstone are found in many countries and places: the European Alps; Brazil; India; Mexico; Myanmar; Madagascar; Sri Lanka; the USA, specifically Pennsylvania and Virginia; and Tanzania. However, it is Sri Lanka that produces the highest quality moonstones.
