16.8.3: Grossular

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gemology
Gemology Project

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Grossular Garnet
Chemical composition	Ca₃Al₂(SiO₄)₃alcium 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 (singly refractive)
Hydrogrossular garnet: 1.70-1.73 (singly 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

Figure \(\PageIndex{1}\): Faceted Mint Garnet
Photo courtesy of Ben Pfeiffer

Green grossular garnet is the bluish-green to yellowish-green variety. Within this group, Merelani Mint garnet is a grossular variety that is found in the Merelani Hills of Tanzania. It has emerged in the last few years with a bright pastel mint green color. Due to its limited availability, it has remained mainly as a collector stone.

Hydrogrossular garnet

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 a grossular that, like the Meralani mint, is green in colour. It is coloured by chromium and/or vanadium. The distinction between a 'regular' green grossular and tsavorite is commonly thought to be a function of saturation and tone. However, according to the Bridges' family (tsavorite was discovered by the late Campbell Bridges), the distinction is chemical. The chromophore for green grossular is iron, as opposed to chromium and/or vanadium.

Mali

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