A feldspar duet

Classic moonstone is an orthoclase–albite intergrowth: potassium feldspar intertwined with sodium feldspar. As the crystal cooled, the two separated into ultra‑thin layers (lamellae). Those layers are the stage for adularescence.

Adularia & “rainbow moonstone”

The term adularia (a low‑temperature orthoclase) comes from the Adula Alps, where pale feldspar with a soft glow was first celebrated. Today you’ll also see “rainbow moonstone”—a trade name for a plagioclase feldspar (labradorite) that shows multicolor flashes (labradorescence). Both are feldspar and both are beautiful, but the cause of color is different.

Lamellae, like pages in a book

During slow cooling, a feldspar crystal unmixes into K‑rich and Na‑rich layers only hundreds of nanometers thick. Light entering the stone scatters and interferes between layers, creating a soft, hovering glow called adularescence.

Why some are blue

Very thin lamellae scatter shorter wavelengths (blue). Slightly thicker lamellae produce white/silver sheen. The best “blue moon” material looks like light trapped in ice.

Cat’s‑eye & stars

Needle‑like inclusions (often rutile) aligned in one direction can create a chatoyant line (cat’s‑eye); two sets at angles may form a delicate four‑ray star on a high dome.

Palette

• Colorless to milky body with blue/white sheen.
• Peach/cream from fine hematite/ilmenite specks (“peach moonstone”).
• Grey to smoky body with silver flash (“black” moonstone).
• Rainbow flashes (in labradorite‑type “rainbow moonstone”).

Best pieces show a centered sheen that glides smoothly under a single light source.

Pattern words

• Adularescent billow — a soft, floating patch of light.
• Eye — a narrow, bright band (chatoyancy) sliding across the dome.
• Star — faint four‑ray cross on a high cabochon.
• Centipede lines — fine, parallel tension lines inside (a natural moonstone quirk).

Photo tip: One small point light held above the cab at ~30° makes the sheen pop. Keep the light still and tilt the stone—moonstone loves a spotlight cue.

Laminae & “centipedes”

At 10×, look for fine parallel lines or “centipede” textures—tiny, ladder‑like stress features running with the lamellae. They’re common and part of the species’ personality.

Plagioclase vs. orthoclase

“Rainbow moonstone” (labradorite) often shows fine polysynthetic twinning striations on flats; classic orthoclase moonstone shows cleaner interiors with billowing sheen rather than sharp spectral color bands.

Tube & plate inclusions

Needly rutile or ilmenite can align to produce a cat’s‑eye; thin platelets may add a soft glitter under the sheen in peach material.

Opalite glass

Man‑made glass with milky blue/orange look. Tells: bubbles, very even color, high clarity, and a plastic‑smooth feel. True moonstone has layered glow, not uniform milkiness.

Opal

Opal shows play‑of‑color (pinpoints/patches that flicker). Moonstone’s glow is a sheet of light that slides smoothly with movement.

Girasol quartz & chalcedony

These can show a soft haze but lack a coherent moving sheen. Hardness is higher (won’t scratch with a knife) and no feldspar cleavage.

Satin spar (gypsum)

Shows a cat’s‑eye band from fibers, not a floating billow; much softer (Mohs ~2), easily scratched by a fingernail.

White labradorite (true “rainbow”)

Brilliant spectral flashes in patches or bars; often crisper colors (blue, green, yellow). Still lovely—just note it’s a plagioclase showing labradorescence.

Quick checklist

• Does a sheet of light move under a single point light?
• Do you see two cleavage directions under magnification?
• No bubbles or plastic‑smooth molded shapes? (Glass giveaway.)

Sri Lanka & India

Sri Lankan material is famous for electric blue sheen in nearly colorless bodies. India produces classic whites, peaches, greys, and black moonstone with soft silver glow—excellent for larger cabochons and beads.

Myanmar, Madagascar & beyond

Myanmar historically yielded fine blue glow stones; Madagascar supplies both classic and “rainbow” types. Feldspar with sheen turns up worldwide—anywhere slow‑cooled feldspar had time to unmix into lamellae.

Everyday care

• Use lukewarm water + mild soap + soft cloth; dry promptly.
• Avoid ultrasonics/steam, extreme heat/cold swings, and hard knocks.
• Store separately—quartz dust can scuff the polish over time.

Jewelry guidance

• Moonstone loves cabs. A protective bezel or halo helps.
• Rings/bracelets: choose thicker domes and wear mindfully; earrings/pendants are easy wins.
• White metals emphasize blue sheen; warm metals flatter peach/cream bodies.

On the wheel

• Orient with lamellae parallel to the base so the sheen floats across the dome.
• Keep coolant steady; avoid heating (cleavage + stress = chips).
• Pre‑polish 1200→3k→8k; finish with cerium or alumina on a firm yet forgiving pad. A high dome concentrates the glow.

Single‑light slide

Place a cab under a phone flashlight. Keep the light still and tilt the stone. The glow should drift like a cloud. If it flickers as sharp color bars, you’re likely holding “rainbow” (labradorite).

Cleavage corners

With a loupe, find micro‑chips on the girdle: feldspar’s two cleavages meet at near right angles—a quick mineral ID that separates it from glass and quartz.