mineral
Opal (Precious Opal)
Hydrated Silica (SiO2·nH2O)
Hardness: 5.5-6.5 Mohs; Color: Multi-colored play-of-color on a white, gray, or clear body; Luster: Subvitreous to pearly; Crystal structure: Amorphous (mineraloid); Cleavage: None; Specific Gravity: 1.9-2.3.
- Hardness
- 5
- Color
- Multi-colored play-of-color on a white, gray, or clear body
- Luster
- Subvitreous to pearly
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Physical properties
Hardness: 5.5-6.5 Mohs; Color: Multi-colored play-of-color on a white, gray, or clear body; Luster: Subvitreous to pearly; Crystal structure: Amorphous (mineraloid); Cleavage: None; Specific Gravity: 1.9-2.3.
Formation & geological history
Formed from silica-rich water seeping into cracks and cavities in sedimentary rocks (like sandstone) or volcanic rocks. Most precious opals formed between 65 and 145 million years ago (Cretaceous period).
Uses & applications
Primarily used in high-end jewelry as gemstones (cabochons or faceted) and highly sought after by mineral collectors.
Geological facts
The 'play-of-color' is caused by the diffraction of light through microscopic silica spheres arranged in a regular grid. Opals are the birthstone for October and over 90% of the world's supply comes from Australia.
Field identification & locations
Identify by its unique internal flashes of color that shift when the stone is moved. Commonly found in Coober Pedy (Australia), Ethiopia, and Mexico. Collectors look for 'fire' intensity and body tone.