Moonstone is a member of the feldspar group. This gem is always cut in cabachon to give a nice moonshine colour hence the name.
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 giving the effect of moonlight.
It has a mohls hardness scale of 6-6.5
and a refractive index of 1.518-1.526.