Moonstone

Moonstone is a gem variety of feldspar prized for adularescence, a soft billowy sheen of light (often blue or silvery white) that appears to float beneath the surface and shift as the stone moves. The effect resembles moonlight, giving the stone its name.

Classic moonstone is an alkali feldspar (orthoclase intergrown with albite), though the trade name rainbow moonstone actually refers to a labradorite-type plagioclase feldspar with multicolored sheen. The finest material is nearly colorless and transparent with a strong blue adularescence.

Moonstone has been treasured since antiquity and was especially popular during the Art Nouveau period.

Moonstone forms in igneous rocks, particularly granites and pegmatites, where two feldspar species (orthoclase and albite) crystallize together and then separate into alternating microscopic layers as the mineral cools. Light scattering between these thin layers produces the adularescent glow.

The name adularia derives from Mount Adular in Switzerland, an early source. Today the most important deposits are in Sri Lanka (fine blue sheen material), India (rainbow and multicolored), Myanmar, Madagascar, Tanzania, Australia and the USA.

Moonstone is recognized by its floating blue to white sheen (adularescence) that moves across the stone; the body is typically colorless, white, gray or peach. Hardness is 6-6.5 and, being feldspar, it has two directions of cleavage, so it can chip. Streak is white.

Look-alikes: Opalite is a man-made glass that shows blue glow but with bubbles and a uniform look. Chalcedony and milky quartz lack the directional floating sheen. Selenite (a gypsum) is much softer (hardness 2) and is unrelated despite name confusion. The directional, depth-floating glow plus feldspar cleavage confirms moonstone.

Moonstone is a popular jewelry gem set in rings, pendants and earrings, usually cut as cabochons to maximize the adularescent sheen. Transparent stones with vivid blue glow are the most valuable; rainbow moonstone is widely used for its multicolored flash.

It is an alternative June birthstone and has long carried associations with the moon, intuition and femininity. Because of its cleavage and moderate hardness it is best for pendants and earrings rather than rough-wear rings.

Metaphysically it is linked to new beginnings, intuition and emotional balance, though such claims are not scientifically supported.