Moonstone Identification Guide

What Moonstone Looks Like

Moonstone is a gem feldspar (an intergrowth of orthoclase and albite) famous for adularescence — a soft, billowy blue or white glow that seems to float beneath the surface and shifts as you tilt the stone. The body is usually colorless to milky white, gray, or peach, and is semi-translucent with a slightly cloudy interior.

• Color: colorless, white, gray, peach, with blue/silver sheen
• Optical effect: adularescence — a moving cloud of light (schiller)
• Transparency: translucent to semi-transparent
• Habit: typically cut as cabochons; rough shows blocky feldspar cleavage

Step-by-Step Field ID Checklist

1. Tilt for the glow. Rock the stone under a single light — true moonstone shows a soft sheen that glides across the surface, not fixed sparkle.
2. Check hardness: Mohs 6–6.5 — it scratches glass weakly and can be scratched by quartz/topaz.
3. Look for cleavage. Feldspar has two good cleavages near 90°; rough pieces split into blocky steps.
4. Note the body. A slightly milky, translucent interior with internal "centipede" cleavage cracks is typical of natural moonstone.
5. Heft and feel: lighter than quartz (SG ~2.56) and cool, glassy.

Key Diagnostic Tests

• Hardness: 6–6.5.
• Cleavage: two directions intersecting near 90° (diagnostic — glass imitations have none).
• Specific gravity: ~2.55–2.61.
• Adularescence: caused by light scattering off lamellar feldspar intergrowths; the effect moves with the stone.
• Inclusions: stress cracks ("centipedes") common in natural stones.

Common Look-Alikes and How to Tell Them Apart

• Opalite (man-made glass): shows a milky blue glow but has gas bubbles, no cleavage, and a conchoidal glassy fracture; it looks "too perfect."
• Chalcedony/milky quartz: harder (7), no cleavage, and lacks the floating sheen.
• White labradorite/rainbow moonstone: rainbow moonstone is actually labradorite-feldspar showing multicolor flashes rather than a single blue cloud.
• Cultured opal/plastic: much softer; plastic warms quickly and is scratched easily.
• Synthetic spinel "moonstone": lacks true cleavage and shows different sheen.

Where Moonstone Is Found

The finest blue-sheen moonstone comes from Sri Lanka and southern India; additional sources include Myanmar, Madagascar, Tanzania, Brazil, and the United States. It forms in granites, pegmatites, and metamorphic rocks where orthoclase and albite exsolve into the fine layers that create its glow.

Forms, Treatments, and Field Notes

Moonstone is sold almost exclusively as cabochons, beads, and carvings because the cabochon dome is what concentrates the adularescent glow. Orient a cab so the sheen "rolls" across the high point as you tilt it — that movement is hard for imitations to reproduce. Natural stones often contain fine internal cleavage cracks (called "centipedes") and a faintly cloudy body; flawless, glassy clarity with a too-perfect blue glow suggests glass or assembled imitation.

Treatments and quality

Most moonstone is untreated, though some is coated or backed with foil to deepen the blue sheen — inspect the base of cabs for coatings. The finest grade, "blue moonstone," combines near-colorless transparency with a vivid blue glow and commands premium prices; white "rainbow moonstone" (labradorite) shows multicolor flashes instead. A hardness of 6–6.5 plus visible feldspar cleavage reliably separates real moonstone from harder quartz and from soft plastic fakes.