Grandidierite is a mineral that was first discovered in 1902 in southern Madagascar. The mineral was named in honor of French explorer Alfred Grandidier (1836-1912) who studied the natural history of Madagascar.
A rather rare mineral, blue-green grandidierite is seldom seen in gem or jewelry collections. Translucent material is sometimes cut into cabochons. Faceted pieces are rare. As cabochons, translucent grandidierite can range from 1 to 10 carats and up to about one inch in size. Most material is generally opaque.
Since its discovery in 1902, most grandidierite used as a gem has been translucent. Recent discoveries of transparent material in Sri Lanka (around 2000) and a new deposit in Madagascar (2014) have resulted in faceted gemstones.
Grandidierites appear bluer in color the more iron (Fe) they contain. A recently discovered gemstone, blue ominelite is the Fe-analogue (Fe, Mg) to grandidierite (Mg, Fe).
Grandidierites display strong trichroic pleochroism. That means they can show three different colors depending on the viewing angle: dark blue-green, colorless (sometimes a very light yellow), or dark green.
While trichroism can usually help distinguish grandidierites from other gems, lazulites can occur with blue-green colors and show colorless/blue/dark blue pleochroism. They also appear cabbed and, more rarely, faceted. Nevertheless, lazulites have somewhat higher refractive indices and specific gravity. Grandidierites also have greater hardness.