mineral
Common Opal
Hydrated amorphous silica (SiO2·nH2O)
Hardness: 5.5 to 6.5 on the Mohs scale. Color: White, yellow, brown, grey, or translucent with waxy to vitreous luster. Crystal structure: Amorphous (lacking a definite crystal structure). Specific gravity: 1.9 to 2.3.
- Hardness
- 5
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Physical properties
Hardness: 5.5 to 6.5 on the Mohs scale. Color: White, yellow, brown, grey, or translucent with waxy to vitreous luster. Crystal structure: Amorphous (lacking a definite crystal structure). Specific gravity: 1.9 to 2.3.
Formation & geological history
Formed at low temperatures from silica-rich water solutions that deposited in cavities of sedimentary, igneous, or metamorphic rocks.
Uses & applications
Used primarily in lapidary work, ornamental carvings, and as a collector's specimen.
Geological facts
Unlike precious opal, common opal typically lacks the vibrant play-of-color (opalescence), though it can still exhibit attractive translucence and banding.
Field identification & locations
Often found in regions with volcanic activity or sedimentary basins rich in silica; identified by its conchoidal fracture and lack of cleavage.
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