mineraloid

Precious Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 Mohs; Color: White/creamy with play-of-color; Luster: Vitreous to pearly; Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3

Hardness: 5
Color: White/creamy with play-of-color
Luster: Vitreous to pearly

Identified June 28, 2026More mineraloid →

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Physical properties

Hardness: 5.5-6.5 Mohs; Color: White/creamy with play-of-color; Luster: Vitreous to pearly; Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary or volcanic rocks. Over millions of years, the water evaporates, leaving behind silica deposits. Many famous deposits are from the Cretaceous period.

Uses & applications

Primarily used as a gemstone in jewelry (rings, pendants, earrings) and as high-value collector specimens.

Geological facts

Opal is the birthstone for October. Its distinctive 'play-of-color' is caused by the diffraction of light by internal sub-microscopic spheres of silica. Australia produces about 95 percent of the world's precious opal.

Field identification & locations

Identify by its unique iridescent flashes (fire). Commonly found in Australia (Coober Pedy), Ethiopia, and Mexico. Collectors should look for vibrancy of color and lack of cracks (crazing).