mineraloid

Common Opal

SiO2·nH2O (Hydrated Silicon Dioxide)

AI-generated identification · may be incorrect

Hardness: 5.5-6.5 on the Mohs scale, Color: White, brown, and translucent variations, Luster: Vitreous to waxy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None, Specific Gravity: 1.9-2.3

Hardness: 5

Identified August 6, 2026More mineraloid →

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Physical properties

Formation & geological history

Formed from a solution of silicon dioxide in water, typically deposited at low temperatures in the cracks and cavities of various sedimentary, igneous, and metamorphic rocks.

Uses & applications

Used primarily in lapidary work, jewelry, and as a mineral specimen for collectors.

Geological facts

Unlike precious opal, common opal (sometimes called potch) generally does not display play-of-color, though it can still exhibit attractive translucency and banding.

Field identification & locations

Identified by its conchoidal fracture, lack of crystal faces, and lower specific gravity compared to quartz. Common worldwide, particularly in Australia, Mexico, and the western United States.