gemstone

Opal

Hydrated amorphous silica, SiO2·nH2O

Hardness of 5.5 to 6.5 on the Mohs scale, typically displays a play-of-color (iridescence) due to the diffraction of light by microscopic silica spheres, subvitreous to pearly luster, and lacks a crystalline structure (amorphous).

Identified August 2, 2026More gemstone →

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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 over millions of years as the water evaporated.

Uses & applications

Primarily used as a gemstone in fine jewelry such as rings, necklaces, and earrings, as well as valued by mineral collectors.

Geological facts

Precious opal contains tiny silica spheres arranged in a regular grid, which causes light to bend and split into the rainbow of colors seen on the surface.

Field identification & locations

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