Grandidierite

Grandidierite is a rare 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.[2]

A large, faceted grandidierite, a 429.87 carat stone owned by a Filipino collector Benedicto G. Membrere V. GIL Report certificate number STN201811139168.

Grandidierite
Tabular crystals of grandidierite from the type locality in Madagascar
General
Category	Nesosilicate
Formula (repeating unit)	(Mg,Fe2+)Al3(BO3)(SiO4)O2
IMA symbol	Gdd[1]
Strunz classification	9.AJ.05
Dana classification	54.01.01.01
Crystal system	Orthorhombic
Crystal class	Dipyramidal H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm
Identification
Color	Cyan to turquoise; bluish green
Crystal habit	Tabular
Twinning	none
Cleavage	Good
Fracture	Brittle
Mohs scale hardness	7.5
Luster	Vitreous, glassy
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.85 to 3.00
Density	2.976
Optical properties	Biaxial (-)
Refractive index	1.583 - 1.639
Pleochroism	Visible
2V angle	Measured: 24° to 32°, calculated: 32°
Dispersion	strong r < v
References	[2][3][4]

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).[5]

Grandidierites display strong trichroic pleochroism. That means that it 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. Nevertheless, lazulites have somewhat higher refractive indices and specific gravity. Grandidierites also have greater hardness, with a 7.5 on the Mohs scale.

Large transparent faceted grandidierite specimens are extremely rare. The largest cut specimen currently known to the GIA weighs in at 763.5 carats.[6]