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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.

Through the polariscope

Moonstone Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Moonstone blinks light-dark four times in a full turn. That is doubly refractive. It does NOT tell you uniaxial from biaxial — that is the optic figure, which needs a conoscope. Computed from optic character and transparency in gems.json. A polariscope separates singly refractive, doubly refractive and aggregate — it does not show the optic sign.

Drawn from the optic character and transparency above. A polariscope separates singly refractive, doubly refractive and aggregate; the optic sign needs a conoscope. How to take this reading.

Specific gravity, and what it rules out

Moonstone SG 2.56–2.59 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 16 other species in the table overlap this range, so a reading here narrows the field to 17 rather than naming the stone. Combine it with a refractive index to separate what is left. Computed from the specific-gravity range in gems.json. Hydrostatic weighing — the bench method this site recommends; heavy liquids are toxic and regulated. Faint ticks are the other species in the table.

Drawn from the specific-gravity range above, against every other species in the table. How to take this reading.

Through the dichroscope

Moonstone One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Moonstone shows no pleochroism, so both windows match at every orientation — turning the instrument changes nothing. That is a real negative result, and the reason it is worth knowing is below. Computed from pleochroism and pleochroic colours in gems.json. Window tints are derived mechanically from the colour words, not chosen; a neutral pair depicts no colour at all.

Drawn from the pleochroism above. Two windows that match mean either no pleochroism or the wrong angle — turn the instrument to tell. How the dichroscope compares.

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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