Moonstone

Moonstone is a variety of orthoclase feldspar that owes its ethereal glow to a phenomenon called adularescence. During crystallization, two types of feldspar - orthoclase and albite - initially form a homogeneous mixture. As the crystal cools, these two components slowly separate into microscopically thin, alternating layers, like shuffled cards.

When light enters the stone and encounters these internal layers, it scatters and interferes, producing a billowy, floating glow that seems to move beneath the surface as the stone is rotated. The effect is named after the Adular Mountains in Switzerland, where fine moonstone specimens were historically found.

The thickness of the internal layers determines the color of the sheen. Thinner layers produce the prized blue adularescence found in Sri Lankan stones, while thicker layers produce white or silver glows. The finest moonstones are nearly transparent with a strong blue sheen that floats across the domed surface of a cabochon cut.

Moonstone is identified by its distinctive adularescence, a soft, billowy glow that moves across the surface when the stone is rotated under light. This distinguishes it from opal (which shows multicolored flashes), labradorite (which shows sharper, more directional color play), and chalcedony (which has no optical phenomenon).

At hardness 6, moonstone can be scratched by quartz. It has two directions of cleavage, which makes it somewhat fragile for jewelry use. The specific gravity of 2.57 is typical for feldspar. Under magnification, the internal layered structure may be visible as a subtle schiller effect.