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Thulite

Last updated: April 2026

Thulite is a manganese-bearing pink variety of zoisite used primarily in ornamental work and cabochons; it is opaque to translucent with a characteristic rose-pink color from Mn substitution in the zoisite structure.

Thulite has a specific gravity of 3.20 and a refractive index of 1.691–1.704, with a Mohs hardness of 6.0–7.0. It is biaxial +.

Physical & Optical Properties

RI Range1.691–1.704
SG Range3.10–3.38
SG Typical3.20
Hardness (Mohs)6–7
Crystal SystemOrthorhombic
Optic CharacterDR Biaxial (+)
Birefringence0.009
Dispersion0.012
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismWeak
ColorsRed Pink
SpeciesZoisite
VarietyThulite (Manganese-Bearing Pink Zoisite)
Red 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

Thulite reads 1.693 – 1.702 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, 0.009 apart at their widest separation. 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

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

Thulite SG 3.10–3.38 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 22 other species in the table overlap this range, so a reading here narrows the field to 23 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

Thulite Trichroic — two of three at a time BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Thulite shows pink in one window and pale pink 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. Thulite 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.

Tanzanite

Key Differentiators

Common Simulants

Treatments

Measurement Guides

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

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

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