mineral

Opal

Hydrated amorphous silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs; Color: Displaying 'play-of-color' (iridiscence) with blue and green flashes; Luster: Vitreous to pearly; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 1.9-2.3

Hardness: 5
Luster: Vitreous to pearly

Identified June 21, 2026More mineral →

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Physical properties

Formation & geological history

Formed from seasonal rains that leach silica from sandstone and carry it into fissures. As the water evaporates, it leaves behind a silica deposit. Most precious opals are found in rocks dating back to the Cretaceous period.

Uses & applications

Primarily used as a gemstone in jewelry (rings, pendants, earrings) and for metaphysical/collector specimens.

Geological facts

Opals are unique because they can contain up to 21% water. The internal structure of silica spheres diffracts light, creating the spectral colors known as 'play-of-color.' Approximately 95% of the world's precious opal comes from Australia.

Field identification & locations

Identify by its characteristic play-of-color and vitreous luster. In the field, look for silica-filled veins in sedimentary or volcanic rock. Often found in places like Coober Pedy, Australia, or Ethiopia.