mineral

Opal

Hydrated amorphous silica (SiO2·nH2O)

Hardness: 5.5-6.5 on the Mohs scale. Color: Varies widely, often displaying a play-of-color (iridescence) against a dark or light body tone. Luster: Vitreous to greasy, sub-vitreous. Crystal structure: Amorphous (non-crystalline, gel-like silica spheres). Cleavage: None. Specific gravity: 1.98-2.20.

Hardness: 5
Luster: Vitreous to greasy, sub-vitreous

Identified August 3, 2026More mineral →

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Physical properties

Formation & geological history

Formed from a solution of silicon dioxide and water that seeped into cracks and voids in sedimentary or volcanic rock, precipitating out over millions of years as the water evaporated.

Uses & applications

Used primarily in high-end jewelry (rings, pendants, earrings), as ornamental carvings, and as a prized specimen for mineral collectors.

Geological facts

Precious opal contains microscopic silica spheres arranged in a regular grid, which diffract white light into the spectral colors that make the stone famous.

Field identification & locations

Identified by its characteristic play-of-color and amorphous structure. Major sources include Australia (which produces the vast majority of the world's supply), Ethiopia, Mexico, and the United States.