mineral

Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5; Color: Milky white with opalescence/schiller; Luster: Vitreous to waxy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 1.9-2.3

Hardness: 5
Color: Milky white with opalescence/schiller
Luster: Vitreous to waxy

Identified April 12, 2026More mineral →

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Physical properties

Hardness: 5.5-6.5; Color: Milky white with opalescence/schiller; Luster: Vitreous to waxy; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 1.9-2.3

Formation & geological history

Formed from silica-rich water seeping into cracks and cavities in sedimentary or igneous rocks (such as basalt or rhyolite). It precipitates out over millions of years as the water evaporates.

Uses & applications

Predominantly used as a gemstone in jewelry (rings, pendants, earrings); collector specimens; historically used in some early optical components.

Geological facts

Opal is a 'mineraloid' rather than a mineral because it lacks a crystalline structure. It contains between 3% and 21% water. 95% of precious opal comes from Australia.

Field identification & locations

Identify by its waxy luster and unique play-of-color (opalescence) when tilted. In the field, look for 'potch' (common opal) in desert environments or volcanic regions where hot springs once occurred.