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Sunstone

Last updated: April 2026

A plagioclase feldspar (typically oligoclase or andesine) displaying aventurescence — a metallic shimmering or spangling caused by flat metallic inclusions. Oregon sunstone (often andesine-labradorite, RI up to ~1.568) may show red, green, or bicolor zones. Indian sunstone (oligoclase) shows a golden glitter from hematite/goethite platelets. Copper-bearing Oregon sunstones are natural and untreated.

Sunstone has a specific gravity of 2.63 and a refractive index of 1.537–1.572, with a Mohs hardness of 6.0–6.5. It is biaxial −.

Physical & Optical Properties

RI Range1.537–1.572
SG Range2.62–2.65
SG Typical2.63
Hardness (Mohs)6–6.5
Crystal SystemTriclinic
Optic CharacterDR Biaxial (−)
Birefringence0.010
Dispersion0.012
Fluorescence LWWeak orange
Fluorescence SWInert
Chelsea FilterInert
PleochroismWeak
ColorsYellow Orange, Red Pink, Colorless, Green
SpeciesFeldspar
VarietyPlagioclase (Oligoclase / Andesine)
Yellow OrangeRed PinkColorlessGreen

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

Sunstone varies 1.537 – 1.572 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, 0.010 apart at their widest separation. Thepair drawn here is one specimen: this species is publishedat 1.537 – 1.572, and the two edges slide together anywhereacross it, between the dashed marks. MEANSThat gap IS the birefringence — and it is the stable part.Where the pair sits varies by 3 printed divisions from stoneto stone, so the gap identifies and the position onlynarrows. Take max minus min across the whole rotation; asingle position understates it. 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

Sunstone Blinks 4× per turn (DR) 0° 45° 90° 135° ROTATION BETWEEN CROSSED POLARS Sunstone 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

Sunstone SG 2.62–2.65 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 25 other species in the table overlap this range, so a reading here narrows the field to 26 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

Sunstone Trichroic — two of three at a time BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Sunstone shows pale yellow in one window and orange-red in the other. Rotate the instrument 45° and the two windows become IDENTICAL — the null. So "both windows the same" means either the stone is singly refractive or you are holding the instrument at the wrong angle, and the only way to tell is to turn it. Sunstone is trichroic: a third colour, colorless, appears in a different orientation, and a dichroscope only ever shows two at once. 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.

AmazoniteLabradoriteMoonstoneRainbow Moonstone

Key Differentiators

Treatments

Measurement Guides

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

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

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Studying for the GIA, FGA, or FEEG practical? Sunstone 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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