mineral

Rough Precious Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 (Mohs); Color: Multi-colored flashes (play-of-color) on a brownish-yellow base; Luster: Sub-vitreous to waxy; Crystal structure: Amorphous (non-crystalline); Cleavage: None (conchoidal fracture); Specific gravity: 1.9-2.3

Hardness: 5
Luster: Sub-vitreous to waxy

Identified March 11, 2026More mineral →

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Physical properties

Formation & geological history

Formed from the precipitation of silica-rich water in cavities and cracks of igneous or sedimentary rocks. This specimen appears to be an Ethiopian or Australian 'chocolate' or 'matrix' opal, often dating from the Cretaceous to Miocene periods.

Uses & applications

Primary use is in the gemstone and jewelry industry. High-quality pieces are cut into cabochons. Lesser specimens are kept as rough mineral samples for collectors.

Geological facts

Opals are unique because they contain up to 20% water trapped within their silica structure. The 'fire' is caused by the diffraction of light through tiny, regularly spaced silica spheres.

Field identification & locations

Identify by looking for 'play-of-color' (iridescent flashes of green, red, or yellow) when rotated under light. Commonly found in Australia (Coober Pedy/Lightning Ridge) and the Wollo Province of Ethiopia. Field identification involves checking for a waxy texture and spectral colors.