Star Moonstone Identification Guide
Identify star moonstone by its feldspar adularescence plus a four-rayed star or cat's-eye, low hardness, cleavage, and feldspar look-alikes.
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What Star Moonstone Looks Like
Star moonstone is a rare feldspar moonstone (usually orthoclase, or orthoclase-albite intergrowth) that combines the milky-blue floating glow called adularescence with asterism — a star, typically four-rayed, or a single chatoyant (cat's-eye) band. The body is translucent white to gray with a billowy blue or silvery sheen that seems to float beneath the surface, plus the crossed light bands of the star.
- Color: white to gray, with blue/silver adularescent sheen
- Luster: vitreous to pearly
- Transparency: translucent to semi-transparent
- Habit: cut as cabochons (the dome is needed for both sheen and star)
Step-by-Step Field-ID Checklist
- Tilt the cabochon under a single light — first confirm the billowy blue/white adularescence that drifts as you move it (the moonstone hallmark).
- Look for the star or eye. A four-rayed star or a single sharp light line indicates asterism/chatoyancy from oriented inclusions.
- Check the body — soft, slightly cloudy translucency, not the vivid spectral flash of labradorite.
- Test hardness — moonstone is only ~6, so it is scratched by quartz and a hard file.
- Find a cleavage edge — feldspar shows two cleavage directions near 90°.
Key Diagnostic Tests
- Mohs hardness: 6–6.5 — will not scratch quartz; relatively soft.
- Streak: white.
- Cleavage: two perfect/good directions at ~90° (feldspar) — diagnostic versus quartz imitations.
- Density: ~2.56–2.62 g/cm³, light.
- Refractive index: ~1.52–1.53, low.
- Adularescence: the floating blue/white sheen is the defining optical effect.
- No acid reaction; not magnetic.
Common Look-Alikes and How to Tell Them Apart
- Star/cat's-eye chalcedony or quartz: quartz is harder (7), has no cleavage, and lacks adularescence; moonstone is softer with cleavage and the floating glow.
- Milky/girasol glass imitations: glass shows a uniform glow and gas bubbles, no true adularescence, no cleavage, and feels warm.
- Rainbow moonstone (labradorite): shows multicolor flash, not the single blue sheen, and seldom asteriated.
- Star sapphire: far harder (9), denser, six-rayed star, no adularescence.
- Chatoyant moonstone (cat's-eye moonstone): same material with a single line rather than a crossed star — distinguished by ray count.
Where It Is Found
Fine moonstone, including the rare asteriated material, comes principally from Sri Lanka (the classic blue adularescent source) and India, with additional feldspar moonstone from Myanmar and Madagascar. It occurs in pegmatites and metamorphic rocks and is recovered from gem gravels.
Frequently asked questions
How can you tell if it's real star moonstone?
Genuine star moonstone is feldspar: hardness ~6–6.5, low density (~2.6), two cleavage directions near 90 degrees, and a floating blue-white adularescent sheen, with an added four-rayed star or cat's-eye line. Glass fakes glow uniformly, show bubbles, have no cleavage, and feel warm.
What is adularescence in moonstone?
It is the soft, billowy blue-to-white light that appears to float just beneath the surface, caused by light scattering between microscopic layers of intergrown feldspar.
How many rays does a star moonstone have?
Star moonstone typically shows a four-rayed star, or sometimes a single chatoyant (cat's-eye) band, rather than the six-rayed star seen in corundum.
Star moonstone vs star sapphire — how do I tell them apart?
Star sapphire is much harder (9), denser, and shows a six-rayed star with no adularescence; star moonstone is softer (6–6.5), lighter, four-rayed, and displays the floating blue moonstone glow.
Is star moonstone rare?
Yes. Moonstone that shows true asterism in addition to adularescence is uncommon and comes mainly from Sri Lanka and India.