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Demantoid is a rare, vibrant green gemstone and the most valuable variety of andradite garnet. Its name, meaning "diamond-like," comes from its exceptional brilliance and "fire," which is a result of its high dispersion. Key features include its green to yellowish-green color, which can be as saturated as fine emeralds, and characteristic "horsetail" inclusions, which are delicate, golden, thread-like patterns that can increase a stone's value. It has a Mohs hardness of 6.5 to 7.0 and was first discovered in Russia's Ural Mountains.

Physical and optical properties

• Color: A green to yellowish-green hue, prized for its vibrancy.

• Luster: Adamantine, or diamond-like, due to its high brilliance.

• Dispersion: High dispersion of 0.057, which gives it exceptional "fire" or a prismatic effect, even more than a diamond.

• Refractive Index: 1.880 to 1.889.

• Hardness: 6.5 to 7.0 on the Mohs scale.

• Crystal System: Cubic.

• Specific Gravity: 3.84.

Inclusions

• "Horsetails": The most famous and valuable inclusions are golden, thread-like patterns of byssolite or chrysotile asbestos.

• These "horsetails" can add unique character and value to a stone.

• Demantoids from Russia are particularly known for containing these inclusions.

Rarity and value

• Demantoid is one of the rarest and most valuable garnets.

• Stones larger than a few carats are extremely rare, making smaller sizes more common in jewelry.

• Its high value is attributed to its stunning color, exceptional brilliance, and rarity.

Historical and geographical context

• Discovery: First discovered in the Ural Mountains of Russia in 1851.

• Historical Use: It was used by famous jewelers like Carl Fabergé and Tiffany & Co. during the late 19th and early 20th centuries.

• Locations: Key sources include Russia, Namibia, and Iran.