Mineral

Opal

Hydrated Amorphous Silica (SiO2·nH2O)

Hardness: 5.5–6.5 on Mohs scale; Color: Variable with characteristic 'play-of-color'; Luster: Vitreous to waxy; Crystal Structure: Amorphous (mineraloid); Cleavage: None; Specific Gravity: 1.9–2.3

Hardness: 5
Color: Variable with characteristic 'play-of-color'
Luster: Vitreous to waxy

Identified March 17, 2026More mineral →

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Physical properties

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary or volcanic rock. Over millions of years, the water evaporates, leaving behind silica deposits. Most commercial opal is from the Cretaceous period (about 100 million years ago).

Uses & applications

Primary use is in high-end jewelry (cabochons, beads). Also valued as an October birthstone and by mineral collectors.

Geological facts

Opals are not technically minerals because they lack a crystalline structure; they are 'mineraloids'. The 'play-of-color' is caused by the diffraction of light through tiny silica spheres stacked in a grid-like pattern.

Field identification & locations

Identify by its unique iridescent flashes across the surface. Commonly found in Australia (which produces ~95% of precious opal), Ethiopia, Mexico, and the USA. Collectors should look for vibrancy of color and body tone.