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Garnet (various calcium and aluminum silicates)

Color: Red, brown, orange, yellow, green, blue, violet, colorless and black.

Cleavage: Imperfect

Fracture: Subconchoidal, usually brittle.

Heat sensitivity: Low to none, but tiny flaws such as cracks and bubbles can be exacerbated with heat.  Rhodolite seems to be the most heat sensitive of the garnets.

Pleochroism: None

Birefringence (double refraction): Only in rhodolite.

Crystal structure: Isometric, rhombic, dodecahedron.

--------Pyralspite Subgroup:

Pyrope (magnesium aluminum silicate - occasionally contains chromium and usually some iron colorants.)

Color: Intense deep red.

Moh’s hardness: 7-7.5

Refractive index: 1.73-1.76

Critical angle: 35°

Specific gravity: 3.65-3.8

Dispersion: Moderately high (.022-.023)

Almandite (iron aluminum silicate - iron is the red colorant.)

Color: Deep red with a wine to brown undertone.

Moh’s hardness: 7.5

Refractive index: 1.78-1.83

Critical angle: 34.5°

Specific gravity: 3.95-4.25

Dispersion: Moderately high (.024-.027)

Spessartite (magnesium aluminum silicate - manganese is the orange colorant.)

Color: Orange, red to brown.

Moh’s hardness: 7-7.5

Refractive index: 1.795-1.815

Critical angle: 33.5°

Specific gravity: 4.12-4.2

Dispersion: Moderately high (.027)

Rhodolite (combination of pyrope and almandine - iron is the purple colorant.)

Color: Rose-red, raspberry to violet.

Moh’s hardness: 7-7.5

Refractive index: 1.75-1.77

Critical angle: 34.5°

Specific gravity: 3.95-4.25

Dispersion: Moderately high (.023-.027)

            Birefringence (double refraction): None to slight.

--------Ugrandite Subgroup:

Grossular (calcium aluminum silicate)

Types/Color:

• Tsavorite - Green - vanadium is the green colorant.

• Leuco garnet - Colorless

• Hessonite - Brown-orange, peachy-pink - manganese is the orange/yellow colorant.

Moh’s hardness: 7

Refractive index: 1.78-1.83

Critical angle: 34.5°

Specific gravity: 3.95-4.25

Dispersion: Moderately high (.024-.027)

Andradite (calcium iron silicate)

Types/Color:

• Demantoid - Green, yellow-green to yellow-brown - chromium and iron are the green colorants.

• Topazolite - Lemon-yellow

• Melanite - Opaque black

Moh’s hardness: 6.5-7

Refractive index: 1. 85-1.899

Critical angle: 32

Specific gravity: 3.82-3.85

Dispersion: Very strong (.057)

Birefringence (double refraction): None to slight.

Cutting: Pre-polishing with a 1200 Nubond or a 3000 standard lap may cause orange peel which makes polishing difficult.

Polishing: Alumina on most common polishing laps.  50,000 diamond on most polishing laps, and 100,000 on ceramic.

Comments: Garnet does not always form in one of the above types.  Many garnets are a mix of different types.  For example Mali garnet, is a combination of andradite and grossular.  Dematoid (andradite) is the most valuable green garnet, and is extremely rare.  Most green garnet is tsavorite (grossular), which is still quite valuable.  A recent find of garnet has photochroic (color change) properties.

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