mineral

Opal

Hydrated amorphous silica (SiO2·nH2O)

Hardness: 5.5–6.5 Mohs scale. Color: Iridescent 'play-of-color' including pink, orange, and blue. Luster: Vitreous to waxy. Crystal structure: Amorphous (non-crystalline). Cleavage: None. Specific gravity: 1.9–2.3.

Hardness: 5
Color: Iridescent 'play-of-color' including pink, orange, and blue
Luster: Vitreous to waxy

Identified March 18, 2026More mineral →

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Physical properties

Formation & geological history

Formed from the precipitation of silica-rich water in the cracks and cavities of sedimentary or volcanic rocks. Most precious opals were formed during the Cretaceous period (approx. 65-145 million years ago).

Uses & applications

Primary use is in high-end and costume jewelry as gemstones; also highly valued as collector specimens.

Geological facts

Opal is the birthstone for October. Unlike most gemstones, opal is a 'mineraloid' because it lacks a crystal structure. Australia produces over 90% of the world's precious opal.

Field identification & locations

Identify by 'play-of-color' (iridescence) and a characteristic waxy surface. Often found in nodules or filling seams in sandstone or basalt. Look for the 'flash' when tilted in sunlight.