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Moonstone

Last updated: April 2026

Characterized by a billowy, bluish-white light effect called adularescence. IMPORTANT: "Rainbow moonstone" is a trade name for white labradorite (plagioclase feldspar), NOT orthoclase moonstone. Distinguish by RI: labradorite reads above 1.559; true moonstone reads 1.518–1.526 on the refractometer. One of the most commonly misidentified feldspars in consumer jewelry.

Moonstone has a specific gravity of 2.58 and a refractive index of 1.518–1.526, with a Mohs hardness of 6.0–6.5. It is biaxial −.

Physical & Optical Properties

RI Range1.518–1.526
SG Range2.56–2.59
SG Typical2.58
Hardness (Mohs)6–6.5
Crystal SystemMonoclinic
Optic CharacterDR Biaxial (−)
Birefringence0.008
Dispersion0.012
Fluorescence LWWeak blue
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsColorless, Yellow Orange, Brown, Green, Red Pink
SpeciesFeldspar
VarietyOrthoclase
ColorlessYellow OrangeBrownGreenRed Pink

Measured with a polariscope, a refractometer, a hydrostatic balance, a Chelsea filter, a UV torch, a loupe, a darkfield loupe and a spectroscope — step-by-step for each.

Through the refractometer

Moonstone reads 1.518 – 1.526 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.75 1.80 REFRACTIVE INDEX DORotate the stone through a full 360°, reading both edges atevery 45°. SEETwo boundaries 0.008 apart — so close that at this size theyread as one thick line. On the instrument they separate onlywith a steady head and a sharp edge. MEANSThat gap IS the birefringence. Take max minus min across thewhole rotation — a single position understates it, so onereading is an incomplete one. Computed from gems.json — edge positions are arithmetic, not drawn. RI increases upward; the field above the boundary is the dark one.

Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.

AmazoniteLabradoriteRainbow MoonstoneSunstone

Key Differentiators

Commonly Confused With

Commonly confused with: Rainbow Moonstone.

Treatments

Measurement Guides

How to Identify a GemstoneHow to Measure Specific GravityHow to Use a RefractometerUV Fluorescence Testing

Identifying a moonstone? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.

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Studying for the GIA, FGA, or FEEG practical? Moonstone is one of the 132 species in Loupewise’s drill and exam pool — practice identifying it from its measured properties under timed exam conditions.

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