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
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
Drawn from the refractive index above, on a 1.40–1.81 scale. How to take this reading.
Key Differentiators
- Extreme trichroism: dark blue-green / colorless / dark green — essentially diagnostic for this gem
- Biaxial negative with notable birefringence 0.033 — birefringence visible under loupe in larger stones
- Lower RI (1.590–1.623) 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 1.762–1.770 (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 (overlapping grandidierite's upper end and extending higher), SG 2.82–3.32; different trichroism pattern (effectively dichroic); Chelsea inert.
- Aquamarine: Aquamarine: uniaxial negative, RI 1.577–1.583 (lower), SG 2.68–2.78 (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.
Measurement Guides
Identifying a grandidierite? Loupewise walks through these tests in order — RI, SG, fluorescence, and more.
Try Loupewise Free →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.
Practice exams →