mineraloid

Precious Opal

Hydrated Silica (SiO2·nH2O)

Hardness: 5.5-6.5 (Mohs); Color: White to translucent base with play-of-color; Luster: Subvitreous to pearly; Crystal structure: Amorphous; Cleavage: None; Specific Gravity: 1.9-2.3

Hardness: 5
Color: White to translucent base with play-of-color
Luster: Subvitreous to pearly

Identified March 25, 2026More mineraloid →

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Physical properties

Hardness: 5.5-6.5 (Mohs); Color: White to translucent base with play-of-color; Luster: Subvitreous to pearly; 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 volcanic rock. Over millions of years (often dating to the Cretaceous period), the water evaporates, leaving behind silica spheres. The play-of-color is caused by the diffraction of light through these orderly spheres.

Uses & applications

Primarily used as a gemstone in high-end jewelry (cabochons). Low-grade opal (potch) is used for industrial abrasives or fillers, though 'precious' opal is reserved for aesthetic use.

Geological facts

Opal contains between 3% and 21% water. It is Australia's national gemstone, producing roughly 95% of the world's supply. Famous varieties include the 'Aurora Australis' and the 'Olympic Australis'.

Field identification & locations

Field identification: Look for the 'play-of-color' (iridescence) and a waxy or glassy feel. Found in Australia (Coober Pedy, Lightning Ridge), Ethiopia, and Mexico. Collectors should look for 'fire' intensity and pattern uniformity.