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Moonstone
General
Category	orthoclase feldspar
Chemical formula	KAlSi3O8
Identification
Color	Can be numerous colors, including grey, white, pink, green and brown, but the most valuable are deep blue.
Crystal system	Monoclinic
Mohs scale hardness	6 - 6.5
Luster	vitreous
Specific gravity	2.55 - 2.58
Optical properties	Biaxial Negative
Refractive index	1.52 - 1.53 to 1.53 - 1.54
Dispersion	0.008

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 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 (Burma)
Madagascar
Sri Lanka
the USA, specifically;
Pennsylvania and Virginia
Tanzania

Sri Lanka produces the highest quality moonstones.

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