mineral

Opal

Hydrated amorphous form of silica (SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale; Color: Variable (white, black, or blue base with internal play-of-color); Luster: Vitreous to waxy; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 1.9-2.3

Hardness: 5
Luster: Vitreous to waxy

Identified February 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed from the deposition of silica-rich water in fissures and cavities of sedimentary or volcanic rocks. Most deposits date from the Cretaceous period to the Tertiary period.

Uses & applications

Primarily used as a gemstone in jewelry (rings, necklaces, earrings). Also highly sought after by mineral collectors for rare color patterns and matrix specimens.

Geological facts

Opal is Australia's national gemstone, which produces approximately 95% of the world's precious opal. The internal structure consists of silica spheres that diffract light, creating 'play-of-color'.

Field identification & locations

Identified by its unique iridescence or 'opalescence'. Commonly found in Coober Pedy (Australia), Ethiopia, and Mexico. Field identification involves looking for waxy luster and internal flashes of light.

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