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Andalusite

Last updated: April 2026

Strong trichroism (green, red-brown, yellow-orange) is the primary diagnostic; color variation is often visible without instruments. The chiastolite variety contains a black cross-shaped pattern of carbonaceous inclusions.

Andalusite has a specific gravity of 3.17 and a refractive index of 1.629–1.649, with a Mohs hardness of 7.0–7.5. It is biaxial −.

Physical & Optical Properties

RI Range1.629–1.649
SG Range3.13–3.21
SG Typical3.17
Hardness (Mohs)7–7.5
Crystal SystemOrthorhombic
Optic CharacterDR Biaxial (−)
Birefringence0.010
Dispersion0.016
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Trichroic
ColorsGreen, Brown, Yellow Orange
SpeciesAndalusite (Al₂SiO₅)
GreenBrownYellow Orange

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

Andalusite reads 1.634 – 1.644 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. 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.

Key Differentiators

Measurement Guides

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

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

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