gemstone

Precious Opal

Hydrated Silicon Dioxide (SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale. Color: Translucent to white body tone with iridescent play-of-color (flashes of neon green, yellow, orange). Luster: Vitreous to sub-vitreous. Structure: Amorphous (mineraloid consisting of microscopic silica spheres). Cleavage: None (conchoidal fracture).

Hardness: 5
Luster: Vitreous to sub-vitreous

Identified July 27, 2026More gemstone →

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Physical properties

Formation & geological history

Formed when silica-rich water percolated into cracks and cavities within sedimentary rocks or volcanic deposits, slowly evaporating over millions of years (commonly Cretaceous age) to leave behind hydrated silica gel that solidified.

Uses & applications

Cut and polished as cabochons or freeform polished gemstones primarily used in fine jewelry, rings, pendants, and prized by gemstone collectors.

Geological facts

The striking play-of-color is caused by internal diffracted light passing through regularly packed sub-microscopic spheres of silica. Precious opal is the national gemstone of Australia, which produces over 90% of the world's supply.

Field identification & locations

Identified by its distinctive iridescent play-of-color flashes under light and relatively low specific gravity/hardness compared to quartz. Common sources include Coober Pedy and Lightning Ridge in Australia, Ethiopia (Welo), and Mexico.