Grandidierite
Grandidierite is an exceptionally rare borosilicate mineral from Madagascar displaying diagnostic strong trichroism in dark blue-green, pale blue, and colorless-to-pale-yellow; faceted transparent specimens are among the most valuable and coveted collector gemstones.
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)
Key Differentiators
- Extreme trichroism: dark blue-green / pale blue / colorless-to-pale-yellow — essentially diagnostic for this gem
- Biaxial negative with notable birefringence 0.037 — birefringence visible under loupe in larger stones
- Lower RI (1.583–1.639) and SG (2.85–2.98) than teal sapphire (RI 1.762, SG 4.00) — very different density
- One of the world's rarest gem minerals — primary source is Anivorano district, Madagascar
- Chelsea filter inert — no Cr or Fe reaction; distinguishes from chrome tourmaline and some green gems
Common Simulants
- Teal Sapphire: Teal sapphire: uniaxial negative DR, RI 1.762–1.778 (much higher), SG 3.99–4.01 (much denser); dichroic not trichroic; easy separation by RI and SG.
- Blue Tourmaline / Indicolite: Tourmaline: uniaxial negative (trigonal), RI 1.614–1.643 (lower range), SG 2.82–3.32; different trichroism pattern (effectively dichroic); Chelsea inert.
- Aquamarine: Aquamarine: uniaxial negative, RI 1.577–1.583 (lower), SG 2.68–2.80 (lower); dichroic (blue-green / near-colorless); typically paler blue with less saturation.
- Blue Zircon: Zircon: uniaxial DR with extreme birefringence (0.036–0.059); RI 1.810–2.024 (much higher); SG 3.90–4.73; strong facet doubling visible under loupe — completely different profile from grandidierite.
Price Context
Natural — low ($/ct)$500
Natural — high ($/ct)$10,000
NotePer carat; transparent faceted material $500–5,000/ct fine; exceptional vivid trichroic stones can exceed $10,000/ct due to extreme rarity; primary source Anivorano district, Madagascar; translucent material much less expensive
Price context is approximate. GemID is not an appraisal tool. Results are indicators, not certified valuations.
About Grandidierite
Grandidierite is an exceptionally rare borosilicate mineral from Madagascar displaying diagnostic strong trichroism in dark blue-green, pale blue, and colorless-to-pale-yellow; faceted transparent specimens are among the most valuable and coveted collector gemstones.
Identifying a grandidierite? GemID walks through these tests in order — RI, SG, fluorescence, and more.
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