mineral

Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs scale; Color: Variable with play-of-color (iridescence); Luster: Subvitreous to pearly; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 1.9-2.3

Hardness: 5
Color: Variable with play-of-color (iridescence)
Luster: Subvitreous to pearly

Identified June 18, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 Mohs scale; Color: Variable with play-of-color (iridescence); Luster: Subvitreous to pearly; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 1.9-2.3

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary or volcanic rock. Over millions of years, the water evaporates, leaving behind solid silica spheres. Common in Cenozoic era deposits.

Uses & applications

Primarily used in jewelry (rings, pendants, earrings), as birthstones, and by lapidary collectors for its unique optical properties.

Geological facts

Unlike most minerals, opal is amorphous (lacks a crystalline structure). Precious opal's rainbow effect is caused by the diffraction of light through tiny, regularly spaced silica spheres.

Field identification & locations

Identify by its characteristic play-of-color and low weight relative to size. Commonly found in Australia (which produces 95% of the world's supply), Ethiopia, and Mexico. Collectors should look for vibrant flashes of red and orange.