Grandidierite are seafoam and deep ocean blue gemstones from a now depleted deposit discovered in 2014 near the town of Tranomaro in southern Madagascar.
Relatively unknown, Grandidierite is extremely scare, often appearing on lists of the worlds’ 10 rarest gemstones.
Refractive Index 1.583 - 1.639
Relative Density 2.85 - 3.0
Grandidierite comes in beautiful transparent greenish blues and rich translucent bluish greens. While small, eye-clean gemstones have been faceted in minuscule quantities, inclusions are a characteristic trait of Grandidierite. Grandidierite is so scarce that gems of almost any size, color or clarity will find a ready market. Grandidierite’s inclusions, needle-like channels, and parallel growth planes makes it challenging to cut. Nevertheless, quality of cut is very important as a skilled lapidary can locate Grandidierite’s inherent eye-visible inclusions in a way that minimizes their impact on beauty.
Grandidierite (Grand-die-deer-ite) was named after French explorer and naturalist Alfred Grandidier (1836–1912) who studied the natural history of Madagascar. An extremely rare magnesium iron aluminous borosilicate, Grandidierite was first discovered at the cliffs of Andrahomana on the southern coast of Madagascar. Grandidierite is bluish green to greenish blue, appearing bluer the more iron it contains. Grandidierite displays trichroic pleochroism, showing three different colors depending on the viewing angle: dark blue-green, colorless (sometimes very light yellow), or dark green. Since its discovery in 1902, most gem-quality Grandidierite has been translucent. Cabochons cut from these crystals look very attractive with some even appearing ‘Jade-like’. Recent discoveries of transparent crystals in Sri Lanka’s Kolonne region (2000) and a new deposit in Madagascar (2014) have resulted in beautiful, faceted gemstones.
In addition to Madagascar, Grandidierite deposits exist in Algeria, Antarctica, Canada, Czech Republic, India, Italy, Malawi, New Zealand, Norway, Sri Lanka, Suriname, and the United States. Despite these discoveries, facetable Grandidierite larger than a millimeter has only been found in Madagascar and Sri Lanka.
The location of its discovery was visited in 1960 by C. Mignot, who was unable to find additional Grandidierite as the small deposit was depleted. Since then, Grandidierite has been mined in a few other places in southern Madagascar, but these have also since depleted. A new deposit of Grandidierite was discovered in May 2014 about 15km from the village of Tranomaro. The area is in the Amboasary district of southern Madagascar’s Anosy region, 60km northwest of Cape Andrahomana. Access from Tolanaro is via a 60km paved road west to Amboasary Atsimo, followed by a rough, unpaved 50km road north to Tranomaro that requires a four-wheel-drive vehicle. Final access from the village of Tranomaro to the deposit is half a day by foot only. Security is a problem because bandits operate throughout the region.
Mining was done by hand due to the deposit’s remote location and limited production. The deposit covered a few acres where the weathered pegmatite (intrusive igneous rock composed of interlocking crystals) was worked by near-surface artisanal and small-scale mining using spades and pickaxes. About 12 miners dug holes up to a depth of 15m and these open-air corridors cross-cut two Grandidierite-bearing veins separated by 30cm to a few meters. The rough crystals were manually extracted and sorted on-site, with the workers carefully removing the valuable mineral specimens to avoid damaging them. Although now depleted, this deposit has provided high-quality Grandidierite for museums and collectors. Grandidierite is also totally natural and unenhanced.
Durability & Care
Grandidierite (Mohs’ Hardness: 7.5) is an excellent choice for everyday jewelry. Grandidierite should always be stored carefully to avoid scuffs and scratches. Clean with gentle soap and lukewarm water, scrubbing behind the gem with a very soft toothbrush as necessary. After cleaning, pat dry with a soft towel or chamois cloth.