mineral
Common Opal
Hydrated silica (SiO2 "n"H2O)
AI-generated identification · may be incorrectHardness: 5.5-6.5 (Mohs scale); Color: Pale green, yellowish-green; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 1.9-2.3
- Hardness
- 5
- Color
- Pale green, yellowish-green
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 5.5-6.5 (Mohs scale); Color: Pale green, yellowish-green; Luster: Vitreous to waxy; Structure: Amorphous (non-crystalline); Fracture: Conchoidal; Specific Gravity: 1.9-2.3
Formation & geological history
Formed from silica-rich groundwater that precipitates in cracks and cavities of sedimentary or volcanic rocks. This specimen likely formed in a volcanic environment where hydrothermal fluids cooled slowly.
Uses & applications
Used primarily as decorative lapidary material, for carving, or as collector specimens. Occasionally used in low-cost jewelry when colorful.
Geological facts
Unlike "precious" opal, common opal does not show a play-of-color (iridescence) because its silica spheres are not arranged in a perfectly ordered grid. Green common opal is often called "pistachio opal" or "prase opal."
Field identification & locations
Identify by its characteristic waxy luster and shell-like (conchoidal) fracture patterns. It feels lighter than most rocks of its size due to water content. Common in Oregon, Idaho, and Australia.
