Grandidierite

Grandidierite occurs in pegmatites, contact-metamorphic rocks and high-grade metamorphic rocks; about 40 localities were known by 2003. The type locality is Cap Andrahomana near Taolagnaro (Fort Dauphin) in southern Madagascar, which for many years was the only well-known source of gem material. Transparent gem material has since come from the Kolonne area of Sri Lanka (around 2000) and from Tranomaro, Madagascar (2014). Other occurrences include Algeria, Antarctica, India, Italy, Malawi, New Zealand, Norway, Suriname and the United States.

Grandidierite is recognized by its blue-green color, strong trichroism (dark blue-green, colorless or very light yellow, and dark green) and refractive index of about 1.583 to 1.639 with birefringence near 0.038. Lazulite can look similar and also shows strong pleochroism, but it has higher refractive indices and specific gravity and is softer. Translucent cabochons may look jade-like. No treatments are known, and synthetic grandidierite has been made only for research, not for jewelry.

At hardness 7.5 grandidierite resists scratching, but its cleavage and brittleness make it vulnerable to blows and chipping, and some stones contain mineral inclusions. Avoid mechanical cleaning systems; use a soft brush, mild detergent and warm water.

Alfred Lacroix described grandidierite in 1902 from Andrahomana in southeastern Madagascar and named it after Alfred Grandidier (1836–1912), the French explorer and naturalist who studied Madagascar's natural history and geography. For a century only opaque or at best translucent material was known; in 2003 Gems & Gemology described the first transparent faceted grandidierite, a 0.29 ct stone cut from a 0.85 ct crystal fragment bought in Ratnapura, Sri Lanka, in 2000.