Mineral

Opal

Hydrated Amorphous Silica (SiO2·nH2O)

Hardness: 5.5-6.5 on Mohs scale; Color: Multi-colored with internal 'play of color' including blue, green, and white; Luster: Vitreous to waxy; Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 1.9-2.3.

Hardness: 5
Luster: Vitreous to waxy

Identified March 13, 2026More mineral →

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Physical properties

Formation & geological history

Formed from a solution of silicon dioxide and water that seeps into cracks and cavities in earth. As the water evaporates, it leaves behind a silica deposit. This specimen appears to be 'Boulder Opal' or 'Opal in host rock' formed in sedimentary or volcanic environments around 5-30 million years ago.

Uses & applications

Primarily used as a gemstone in jewelry (rings, pendants) and highly valued by mineral collectors for its unique optical properties.

Geological facts

Opal is the birthstone for October. Unlike most minerals, it contains 3% to 21% water. The 'play of color' is caused by the diffraction of light as it passes through tiny, orderly-arranged silica spheres.

Field identification & locations

Identify by its unique iridescent play of color and conchoidal (shell-like) fracture. Commonly found in Australia (which produces 95% of the world's supply), Ethiopia, and Mexico. Collectors should look for vibrancy of color and the thickness of the opal layer.