Moonstone
A feldspar gem with a floating blue-white glow — adularescence, the "moonlight" sheen.
Also known as: Adularia
intermediate Feldspar-bearing igneous rocks
What it is
Moonstone is a variety of feldspar, usually orthoclase intergrown with albite in fine alternating layers. Light scattering between those layers produces adularescence: a soft, billowy sheen that seems to float across the stone as it moves, resembling moonlight. It is a modest 6 to 6.5 on the Mohs scale and has two cleavage directions, so it is prone to chipping and needs gentle wear.
The most prized moonstones are nearly colourless and transparent with a vivid blue sheen; body colours also include peach, grey and the multicoloured "rainbow" moonstone (actually a labradorite). Sri Lanka and India are classic sources, along with Myanmar and Madagascar. Moonstone is one of the birthstones for June and is generally sold untreated.