mineral
Common Opal
Hydrated Silica (SiO2·nH2O)
AI-generated identification · may be incorrectHardness: 5.5-6.5 Mohs. Color: Greenish-yellow to olive-drab. Luster: Waxy to vitreous. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 1.9-2.3.
- Hardness
- 5
- Color
- Greenish-yellow to olive-drab
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 5.5-6.5 Mohs. Color: Greenish-yellow to olive-drab. Luster: Waxy to vitreous. Crystal structure: Amorphous. Cleavage: None (conchoidal fracture). Specific gravity: 1.9-2.3.
Formation & geological history
Formed from precipitation of silica-rich water in cavities or veins of volcanic rocks or sedimentary environments. It is often found in Tertiary volcanic deposits.
Uses & applications
Mostly used for lapidary work, cabochons for jewelry, and as decorative specimen pieces for mineral collectors.
Geological facts
Unlike 'Precious Opal', Common Opal does not exhibit a play of colors (opalescence). Oregon is famous for high-quality greenish varieties known as 'Owyhee Opal'.
Field identification & locations
Identify in the field by its distinctive waxy luster and smooth, shell-like (conchoidal) surface when broken. It feels lighter than quartz of the same size. Commonly found in basalt or rhyolite vugs.
