Moonstone is a sodium potassium aluminium silicate, with the chemical formula (Na,K)AlSi3O8.
Its name is derived from a visual effect, or sheen, caused by light diffraction within a microstructure consisting of a regular succession of feldspar layers.
Moonstone has been used in jewelry for centuries, including ancient civilizations. The Romans admired moonstone, as they believed it was born from solidified rays of the moon. Both the Romans and Greeks associated Moonstone with their lunar gods and goddesses. In more recent history, the moonstone became popular during the Art Nouveau. French goldsmith, René Lalique and many others created a large quantity of jewelry using the moonstone.
The most common moonstone is of the mineral adularia. The plagioclase feldspar oligoclase also produces moonstone specimens. Moonstone is feldspar with a pearly and opalescent luster. An alternate name is hecatolite.
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 occur in Australia, the Austrian Alps, Mexico, Madagascar, Burma, Norway, Poland, India, Sri Lanka and the United States.
It is currently the state gem for Florida.