Mineraloid
White Opal
Hydrated Amorphous Silica (SiO2·nH2O)
Hardness: 5.5-6.5; Color: White body tone with play-of-color; Luster: Subvitreous to pearly; Crystal structure: Amorphous; Cleavage: None; Specific gravity: 1.9-2.3
- Hardness
- 5
- Color
- White body tone with play-of-color
- Luster
- Subvitreous to pearly
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Physical properties
Hardness: 5.5-6.5; Color: White body tone 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. Most commercial opal deposits formed during the Cretaceous period about 145 to 66 million years ago.
Uses & applications
Primarily used in jewelry as gemstones (cabochons), as well as in high-end watch faces and luxury decorative inlays.
Geological facts
Opals contain between 3% and 21% water. In ancient times, people believed opals possessed the powers of all gemstones whose colors were captured within the opal's play-of-color.
Field identification & locations
Identify by 'play-of-color' (iridescence) and white milky body. Found commonly in Australia (Coober Pedy), Ethiopia, and Brazil. Collectors should look for vibrant flashes of red and green against the white background.
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