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Grandidierite

Last updated: April 2026

Grandidierite is an exceptionally rare borosilicate mineral from Madagascar displaying diagnostic strong trichroism in dark blue-green, colorless (sometimes a very light yellow), and dark green; faceted transparent specimens are among the most valuable and coveted collector gemstones.

Grandidierite has a specific gravity of 2.95 and a refractive index of 1.590–1.623, with a Mohs hardness of 7.0–7.5. It is biaxial −.

Physical & Optical Properties

RI Range1.590–1.623
SG Range2.85–2.98
SG Typical2.95
Hardness (Mohs)7–7.5
Crystal SystemOrthorhombic
Optic CharacterDR Biaxial (−)
Birefringence0.033
Dispersion0.015
Fluorescence LWInert
Fluorescence SWInert
Chelsea FilterInert
PleochroismStrong Trichroic
ColorsBlue Violet, Green
SpeciesGrandidierite ((Mg,Fe)Al₃(BO₃)(SiO₄)O₂)
Blue VioletGreen

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

Grandidierite reads 1.590 – 1.623 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, 0.033 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

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

Grandidierite SG 2.85–2.98 WHERE IT SITS, AND WHAT ELSE IS THERE 1.0 2.0 3.0 4.0 5.0 SPECIFIC GRAVITY 9 other species in the table overlap this range, so a reading here narrows the field to 10 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

Grandidierite Trichroic — two of three at a time BEST ORIENTATION ROTATED 45° — THE NULL At its best orientation Grandidierite shows dark blue-green in one window and colorless 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. Grandidierite is trichroic: a third colour, dark green, 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.

Key Differentiators

Common Simulants

Measurement Guides

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

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

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