Moonstone

Moonstone is a variety of the feldspar-group mineral orthoclase. During formation, orthoclase and albite separate into alternating layers. When light falls between these thin layers it is scattered producing the phenomenon called adularescence. Adularescence is the light that appears to billow across a gem.

Moonstone is a sodium potassium aluminium silicate, with the chemical formula (Na,K)AlSi3O8.

The most common moonstone is of the mineral adularia, named for an early mining site near Mt. Adular in Switzerland, now the town of St. Gotthard. The plagioclase feldspar oligoclase also produces moonstone specimens. Moonstone is feldspar with a pearly and opalescent schiller. An alternative name is hecatolite.

Formation
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.