Grossular

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Grossular Garnet
Chemical composition Ca3Al2(SiO4)3

Calcium aluminum silicate

Crystal system Cubic
Habit Dodecahedra
Cleavage None
Hardness 7.0-7.5
Optic nature Isotropic
Refractive index 1.738-1.745
Birefringence None
Specific gravity 3.60-3.68
Lustre Vitreous

Diagnostics

Color

Green, yellowish-green, yellow-orange and pastel.

Inclusions

Hessonite: granular internal structure from numerous crystal inclusions, giving it an oily appearance.
Green grossular: rod-like or needle-like crystals.
Hydrogrossular: granular structure often with irregular black inclusions.

Refractometer

Hessonite and green grossular: 1.73-1.76 (single refractive)
Hydrogrossular garnet: 1.70-1.73 (single refractive)

Specific gravity

Grossular garnets sink in all common heavy liquids. Hydrogrossular garnet may sink slowly in methylene iodide.

Hessonite and green grossular garnet: 3.60-3.70
Hydrogrossular garnet: 3.35-3.55

Chelsea Colour Filter

Green stones may appear pinkish to red.

Varieties

Hessonite

Hessonite is the brownish-red to yellowish-orange variety.

Green grossular garnet

Green grossular garnet is the bluish-green to yellowish-green variety.

Hydrogrossular garnet

Hydrogrossular garnet cabochon
Photo courtesy of Rick Martin

Hydrogrossular garnet is the green, pink, grayish-white to bluish variety of grossular.

Chrome grossular

Chrome grossular is a green variety of grossular.

Tsavorite

Tsavorite (or tsavolite) is another green variety of chrome grossular.

Mali

Mali garnet is a yellow-green variety of grossular with a slight overlap in the grossular-andradite series.