mineraloid

Opal

Hydrated silica (SiO2·nH2O)

Hardness: 5.5-6.5 (Mohs); Color: White, body with 'play-of-color' (iridescence); Luster: Subvitreous to pearly; Crystal structure: Amorphous (non-crystalline); Cleavage: None; Specific gravity: 1.9 to 2.3

Hardness: 5
Color: White, body with 'play-of-color' (iridescence)
Luster: Subvitreous to pearly

Identified April 14, 2026More mineraloid →

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Physical properties

Formation & geological history

Formed from silica-rich water that seeps into crevices and cavities in rocks. As the water evaporates, it leaves behind a silica deposit. Often found in sedimentary layers or lining volcanic cavities.

Uses & applications

Primarily used in jewelry as gemstones (cabochons or faceted stones) and highly valued by mineral collectors.

Geological facts

Opal is unique because its internal structure consists of tiny silica spheres that diffract light, creating a rainbow-like effect called opalescence. It is the birthstone for October.

Field identification & locations

Identify in the field by its distinct waxy luster and play-of-color. Most famously found in Australia (Coober Pedy, Lightning Ridge) and Ethiopia. Look for colorful flashes when turned under a light source.