Madagascar holds the type locality for grandidierite, a rare magnesium-aluminium borosilicate first described from Andrahomana on the south-east coast in 1902 by the French mineralogist Alfred Lacroix and named for the explorer Alfred Grandidier. For nearly a century the species was known almost exclusively as small, often translucent crystals; transparent gem-quality material was a true rarity until the discovery of new pockets in the Tranomaro and Cap Andrahomana area in the early 2000s and the Tulear region thereafter.

Material character

Grandidierite is strongly trichroic, showing dark blue-green, lighter blue-green and colourless to pale yellow along its three optical directions. Cut stones display a distinctive bluish green to greenish blue body colour, often with a slightly greyish modifier, and the trichroism gives the stones a quiet but unmistakable change of tone as they are tilted. Refractive indices fall between roughly 1.59 and 1.64 with biaxial negative character and a hardness of 7.5, so the gem is durable enough for jewellery use, although cleavage in two directions calls for careful setting.

The bulk of facetable rough comes from a small number of pockets in granulite-facies metamorphic rocks of southern Madagascar. Stones above two carats are scarce, and clean stones above five carats are genuinely rare and command significant premiums.

Trade and identification

Grandidierite is unenhanced; no commercial heat or filling treatment is in use. Trade circulation is limited and mostly concentrated among collectors and specialist dealers. Identification at the laboratory bench is straightforward through refractive index and dichroscope work, but separating genuine grandidierite from blueish-green tourmaline, kornerupine and certain apatite specimens still relies on careful gemmological testing. GIA's Gem Project file and Lotus Gemology have both published reference notes on the species.