Chemical formula: (Na,K)AlSi3O8
Hardness: 6.0 Mohs
Color: blue, grey, white, pink, peach, green, brown
Family: Feldspar variety
Moonstone was the original or traditional birthstone for June before it was changed in the early 1900’s to Alexandrite.
Moonstones were named by the Roman historian Pliny who contended that the stone changed in appearance with the phases of the moon. While this is not precisely correct, like Alexandrite, moonstones do indeed change visual properties in different light. They create an optical phenomena called adularescence which is a scattering of light that typically displays as either a multi-rayed star or a cat’s eye. This occurs because moonstones are formed from two varieties of feldspar that each react slightly differently to light. Moonstones come in a variety of colors including blue, green, peach, and champagne. They are said to possess special properties which grant the wearer good fortune and they are considered a sacred stone in India. Moonstones can be sourced in India, Myanmar, Australia, Madagascar, and the United States.
Moonstone is composed of two feldspar species, orthoclase (KAlSi3O8 ) and albite (NaALSi3O8 ). 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 (a milky, bluish luster or glow originating from below the surface of the gemstone).