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Sugilite

Last updated: April 2026

A rare cyclosilicate mineral found almost exclusively at the Wessels Mine in the Kalahari Manganese Field, Northern Cape, South Africa. Prized for its vivid purple color — top 'gel' quality is translucent with an intense grape-like purple that commands premium prices.

Sugilite has a specific gravity of 2.76 and a refractive index of 1.607–1.610, with a Mohs hardness of 5.5–6.5. It is uniaxial −.

Physical & Optical Properties

RI Range1.607–1.610
SG Range2.74–2.80
SG Typical2.76
Hardness (Mohs)5.5–6.5
Crystal SystemHexagonal
Optic CharacterDR Uniaxial (−)
Birefringence0.002
Dispersion0.010
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismNone
ColorsPurple
SpeciesSugilite
Purple

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

Sugilite reads 1.608 – 1.609 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 0.002 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

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

Sugilite SG 2.74–2.80 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 11 other species in the table overlap this range, so a reading here narrows the field to 12 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

Sugilite One colour (no pleochroism) IDENTICAL AT EVERY ANGLE Sugilite 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.

Key Differentiators

Common Simulants

Treatments

Measurement Guides

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

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

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