Demantoid is an andradite, the iron- and calcium-rich garnet species — formula Ca3Fe2(SiO4)3 — in the larger ugrandite garnet group. Other andradite garnets are topazolite, melanite, and rainbow andradite, but demantoid is the most valuable and widely known andradite variety.

The easiest way to identify demantoids is by the presence of horsetail inclusions (discussed more in-depth in the Clarity section). Not every demantoid has these inclusions, but demantoid is the only green gemstone that has them. Outside of demantoid, they’ve only been found in brown andradite as of 2018.

With a Mohs scale ranking of 6.5 to 7, demantoid’s hardness is the only trait that puts it slightly below other garnets.

On the other hand, demantoid beats other garnets with its diamond-like luster and high dispersion (a.k.a. “fire” or colorful sparkle). The dispersion (0.057) is even higher than that of diamond at 0.044!

Here are all of the demantoid garnet properties:

• Mineral family: Andradite Garnet

• Mohs hardness: 6.5-7

• Color: Yellow-green, emerald-green, or deep green, sometimes brownish tint

• Crystal structure: Isometric/Cubic

• Luster: Adamantine

• Transparency: Transparent

• Refractive index: 1.880-1.888

• Density: 3.81-3.89

• Cleavage: None

• Fracture: Conchoidal

• Streak: White

• Luminescence: None

• Optical effects: Rarely chatoyancy

• TREATMENT NONE

• COLOR GREEN

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