mineral

Common Opal

Amorphous Hydrated Silica (SiO2·nH2O)

Mohs Hardness: 5.5-6.5; Color: White to cream, tan, translucent; Luster: Waxy to vitreous; Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 1.9-2.3

Hardness: 5
Color: White to cream, tan, translucent
Luster: Waxy to vitreous

Identified June 6, 2026More mineral →

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Physical properties

Mohs Hardness: 5.5-6.5; Color: White to cream, tan, translucent; Luster: Waxy to vitreous; Crystal Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 1.9-2.3

Formation & geological history

Formed from the precipitation of silica-rich water in cavities and fissures of sedimentary or volcanic rocks. It is non-crystalline and contains 3% to 21% water by weight. Common opal lacks the 'play of color' found in precious opal.

Uses & applications

Used primarily for lapidary work, cabochons, and as a raw mineral specimen for collectors. In some cases, lower quality common opal is used as an additive in certain industrial filters or abrasives.

Geological facts

Unlike precious opal, common opal is defined by its lack of opalescence. It is sometimes called 'potch' by miners. It is the national gemstone of Australia, although white common opal is found globally.

Field identification & locations

Identified in the field by its waxy luster, conchoidal fracture (smooth, curved breaks), and lower density compared to quartz or chalcedony. It will not scratch glass as easily as quartz. Common in arid regions or volcanic terrains.