mineral

Common Opal (Potch)

Opal (Hydrated silica, SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale; Color: Creamy white, yellow, or tan with a waxy appearance; Luster: Vitreous to waxy; Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.

Hardness: 5
Color: Creamy white, yellow, or tan with a waxy appearance
Luster: Vitreous to waxy

Identified June 24, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 on Mohs scale; Color: Creamy white, yellow, or tan with a waxy appearance; Luster: Vitreous to waxy; Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.

Formation & geological history

Formed from the precipitation of silica-rich water in fissures or cavities of rocks, typically near the earth's surface in volcanic or sedimentary environments. It is often associated with Cenozoic geological ages.

Uses & applications

Used primarily in jewelry as wire-wrapped pendants or tumbled stones. While not as valuable as precious opal, it is popular in crystal healing and amateur lapidary collections.

Geological facts

Unlike precious opal, common opal does not show 'play of color' because its silica spheres are not arranged in a regular, orderly pattern. It contains 3% to 21% water within its structure.

Field identification & locations

Identify by its characteristic waxy luster and shell-like (conchoidal) fracture pattern. Common in arid regions like Australia, Mexico, and the Western United States (Nevada/Oregon).

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