Rock Identifier
Opal (Hydrated amorphous silica (SiO2·nH2O)) — gemstone
gemstone

Opal

Hydrated amorphous silica (SiO2·nH2O)

AI-generated identification · may be incorrect

Hardness of 5.5 to 6.5 on the Mohs scale, vitreous to greasy luster, amorphous crystal structure with no cleavage, and a specific gravity of 2.10 to 2.20. It displays a distinctive play-of-color resulting from the diffraction of light through microscopic silica spheres.

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Physical properties

Hardness of 5.5 to 6.5 on the Mohs scale, vitreous to greasy luster, amorphous crystal structure with no cleavage, and a specific gravity of 2.10 to 2.20. It displays a distinctive play-of-color resulting from the diffraction of light through microscopic silica spheres.

Formation & geological history

Formed from a solution of silicon dioxide and water seeping into cracks and voids in sedimentary or volcanic rock over millions of years.

Uses & applications

Primarily used as a precious gemstone in fine jewelry, rings, necklaces, and collector displays.

Geological facts

Opal contains up to 21% water by weight and can dry out and craze if exposed to extreme heat or low humidity. Australia produces over 90% of the world's supply of precious opals.

Field identification & locations

Identified instantly by its characteristic spectral play-of-color and amorphous structure. Commonly sourced from Australia, Ethiopia, and Mexico.

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