gemstone

Opal

Hydrated silicon dioxide (SiO2·nH2O)

Hardness: 5.5–6.5 on the Mohs scale. Color: Iridescent play-of-color with blue, violet, green, or rainbow flashes on a pale or clear base. Luster: Vitreous to waxy. Structure: Amorphous (non-crystalline matrix of microscopic silica spheres). Cleavage: None.

Hardness: 5
Luster: Vitreous to waxy

Identified July 24, 2026More gemstone →

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Physical properties

Formation & geological history

Formed when silica-rich water seeps into cracks and voids in deep rock layers, slowly evaporating and leaving behind microscopic silica spheres that stack tightly, diffracting light into iridescence.

Uses & applications

Used primarily in fine jewelry including rings, earrings, studs, and pendants.

Geological facts

Opals can contain up to 20% trapped water within their silica structure. Over 90% of the world's precious opal comes from Australia, though significant deposits exist in Ethiopia, Mexico, and Brazil.

Field identification & locations

Identified by its characteristic play-of-color (opalescence), low hardness relative to quartz, and amorphous shell-like fracture. Common in volcanic or weathered sedimentary rock environments.