mineral

Common Opal

Hydrated silica (SiO2 " nH2O)

Hardness: 5.5-6.5 (Mohs scale), Color: Olive green to yellow-green, Luster: Waxy to vitreous, Crystal structure: Amorphous, Cleavage: None (conchoidal fracture), Specific gravity: 1.9-2.3

Hardness: 5

Identified May 3, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 (Mohs scale), Color: Olive green to yellow-green, Luster: Waxy to vitreous, Crystal structure: Amorphous, Cleavage: None (conchoidal fracture), Specific gravity: 1.9-2.3

Formation & geological history

Formed from silica-rich water precipitating in rock cavities and fissures. Often associated with volcanic activity or sedimentary environments where groundwater leaches silica from silicate minerals. Geological age can range from Cenozoic to recent.

Uses & applications

Primarily used for lapidary work such as cabochons, beads, and carvings. Also highly valued by mineral collectors for its unique colors.

Geological facts

While precious opal displays a play-of-color, common opal is opaque to translucent without structural interference colors. Green common opal is often referred to as 'Prase Opal' or 'Pistachio Opal'.

Field identification & locations

Identify in the field by its waxy luster and shell-like (conchoidal) fracture patterns. Found globally, with notable deposits in Tanzania, Serbia, Madagascar, and the Western USA.