mineral

Precious Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs scale; Color: Multi-colored 'play-of-color' (iridescence); Luster: Vitreous to waxy; Crystal structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3; Cleavage: None.

Hardness: 5
Color: Multi-colored 'play-of-color' (iridescence)
Luster: Vitreous to waxy

Identified March 11, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5 Mohs scale; Color: Multi-colored 'play-of-color' (iridescence); Luster: Vitreous to waxy; Crystal structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3; Cleavage: None.

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary rocks (like sandstone) or volcanic rocks. Most Australian deposits formed during the Cretaceous period (145-66 million years ago).

Uses & applications

Primarily used as a gemstone in high-end jewelry (rings, pendants, earrings). Lower grade material is used for carvings or collection as mineral specimens.

Geological facts

Opal is unique because its colors are caused by the diffraction of light through tiny silica spheres. Australia produces about 95% of the world's precious opal. It is the national gemstone of Australia.

Field identification & locations

Identify in the field by the 'play-of-color' that shifts when the stone is tilted under light. Common locations include Coober Pedy and Lightning Ridge in Australia, as well as Ethiopia and Mexico. Collectors should look for vibrancy of color and lack of cracks (crazing).