mineral

Common Opal

Hydrated amorphous silica (SiO2·nH2O)

Hardness: 5.5-6.5 on the Mohs scale, Color: Various shades of white, gray, yellow, brown, or pink without play-of-color, Luster: Vitreous to waxy, Crystal Structure: Amorphous (non-crystalline), Cleavage: None, Specific Gravity: 1.9-2.3

Hardness: 5

Identified August 1, 2026More mineral →

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Physical properties

Formation & geological history

Formed at low temperatures from silica-rich water solutions depositing in crevices, faults, and cavities of various rock types, often associated with volcanic ash deposits over millions of years.

Uses & applications

Used primarily in lapidary work, ornamental carvings, mineral collecting, and occasionally in jewelry when cut as cabochons.

Geological facts

Unlike precious opal, common opal (sometimes called potch) lacks the internal microscopic structure that diffracts light to create a rainbow play-of-color.

Field identification & locations

Identified by its conchoidal fracture, waxy luster, lack of crystal faces, and absence of play-of-color. Commonly found worldwide, especially in Australia, Mexico, and the western United States.