mineraloid
Opal
Hydrated silica (SiO2 · nH2O)
Hardness: 5.5-6.5 Mohs. Color: Iridescent play-of-color (greens, blues). Luster: Subvitreous to pearly. Structure: Amorphous silica spheres.
- Hardness
- 5
- Color
- Iridescent play-of-color (greens, blues)
- Luster
- Subvitreous to pearly
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Physical properties
Hardness: 5.5-6.5 Mohs. Color: Iridescent play-of-color (greens, blues). Luster: Subvitreous to pearly. Structure: Amorphous silica spheres.
Formation & geological history
Formed from silica-rich water seeping into cracks in sedimentary or volcanic rocks, then solidifying over millions of years. Common in Cenozoic deposits.
Uses & applications
Primarily used as a gemstone in jewelry like rings, necklaces, and earrings due to its unique optical properties.
Geological facts
Opals contain 3% to 21% water. Australia produces about 95% of the world's precious opal, though Ethiopia and Mexico are also significant sources.
Field identification & locations
Identify by 'play-of-color' (iridescence) and lack of crystal cleavage. Common in arid sedimentary basins like Coober Pedy or volcanic ridges.
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