mineraloid
Precious Opal
SiO2·nH2O (Hydrated Silicon Dioxide)
Hardness: 5.5 to 6.5 on Mohs scale; Color: Multi-colored iridescence (play-of-color) over a translucent or transparent base; Luster: Sub-vitreous to waxy; Crystal structure: Amorphous (mineraloid); Specific gravity: 1.9 to 2.3
- Hardness
- 5
- Luster
- Sub-vitreous to waxy
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Physical properties
Hardness: 5.5 to 6.5 on Mohs scale; Color: Multi-colored iridescence (play-of-color) over a translucent or transparent base; Luster: Sub-vitreous to waxy; Crystal structure: Amorphous (mineraloid); Specific gravity: 1.9 to 2.3
Formation & geological history
Formed from silica-rich water leaking into deep underground cavities in sedimentary rock formations or volcanic ash layers. As water evaporates over millions of years, silica spheres settle into uniform arrangements, diffracting light into rainbow hues. Major formations date from Cretaceous to Miocene epochs.
Uses & applications
Highly valued as a gemstone for fine jewelry including rings, pendants, and earrings. Exceptional specimens are sought after by high-end mineral and gemstone collectors.
Geological facts
Opals show a unique phenomenon known as 'play-of-color' caused by the internal structure of microscopic silica spheres diffracting white light into spectral colors. Opal is the official birthstone for October. Over 90% of the world's precious opal comes from Australia.
Field identification & locations
Identified by its signature iridescent rainbow flash when exposed to light, combined with a smooth, glass-like fracture and low density. Major sources include Australia (Coober Pedy, Lightning Ridge), Ethiopia (Welo region), Mexico, and the United States.
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