What Is Moonstone? The Optics Behind Its Glow

Moonstone is a feldspar mineral, not a quartz — specifically a variety of orthoclase or a related feldspar exhibiting a distinctive optical effect called adularescence: a soft, billowy glow that appears to float just beneath the surface and shifts as the stone is turned.

The Physics Behind the Glow

Moonstone actually contains two different feldspar types — orthoclase and albite — grown together as microscopically thin, alternating layers within a single crystal. This layered structure forms through a process called exsolution: as the mineral cools slowly deep underground, the two feldspar types, which were initially mixed together at high temperature, gradually separate out into these fine alternating bands as the crystal cools. When the layers are thin enough (on the scale of the wavelength of visible light), they scatter incoming light rather than reflecting it directly off a single surface — producing that soft, diffused, moving glow rather than a sharp reflection.

Moonstone vs. Labradorite

Both belong to the broader feldspar family and both owe their optical effects to internal layering, but the effects themselves are physically distinct. Moonstone's adularescence appears to float diffusely beneath the surface and shifts gently. Labradorite's labradorescence, by contrast, produces sharp, distinct flashes of specific color (often blue, green or gold) that appear and disappear abruptly as the viewing angle changes — closer to thin-film interference (like a soap bubble) than to moonstone's light-scattering effect. The two are sometimes confused by name alone, but the visual effects are quite different once you know what to look for.

Color and Quality

The most prized moonstone shows a blue-toned glow against a colorless, transparent body — this blue sheen commands significantly more than the more common white, grey or peach-toned material. Rainbow moonstone, a trade name for a related feldspar (often actually labradorite or a labradorite-adularia intergrowth) showing multicolored flashes, is a related but technically distinct material from classic blue moonstone.

Where It's Mined

Sri Lanka has historically produced the finest blue moonstone and remains the benchmark source. India is a major current producer of a wider range of moonstone qualities; Myanmar and Madagascar also produce commercial material.

Quick Facts

Mineral family: feldspar (orthoclase/albite intergrowth) — Hardness: 6–6.5 on the Mohs scale — Optical effect: adularescence, from exsolution layering — Most prized source: Sri Lanka.