mineral

Common Opal with Host Rock

Opal (Hydrated silica, SiO2·nH2O)

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

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

Identified May 7, 2026More mineral →

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Physical properties

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

Formation & geological history

Formed from the precipitation of silica-rich water in the cavities or seams of sedimentary or volcanic rocks. This specimen shows a seam of white 'potch' opal attached to its sandstone or rhyolitic host matrix.

Uses & applications

Common opal is used as a semi-precious gemstone in jewelry when it displays attractive colors, though this variety is mostly for mineral collection and decorative display. Silica is also used in abrasives.

Geological facts

Unlike precious opal, common opal does not show 'play-of-color.' It is technically a mineraloid rather than a mineral because it lacks a crystal structure. It can contain up to 20% water within its structure.

Field identification & locations

Identify by its waxy texture, lack of crystal faces, and the way it sits in distinctive layers or veins within a coarser host rock. Common in arid regions like Australia, Ethiopia, and the Western United States.

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