mineraloid
Common Green Opal
Amorphous Hydrous Silica (SiO2·nH2O)
Hardness: 5.5-6.5 on Mohs scale; Color: Olive to lime green; Luster: Waxy to vitreous; Structure: Amorphous (no crystal system); Cleavage: None (conchoidal fracture); Specific Gravity: 1.9-2.3
- Hardness
- 5
- Color
- Olive to lime green
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 5.5-6.5 on Mohs scale; Color: Olive to lime green; Luster: Waxy to vitreous; Structure: Amorphous (no crystal system); Cleavage: None (conchoidal fracture); Specific Gravity: 1.9-2.3
Formation & geological history
Formed from silica-rich waters precipitating in low-temperature environments, often in volcanic rock cavities or sedimentary fissures. Unlike precious opal, it lacks the internal structure to diffract light into play-of-color.
Uses & applications
Used primarily for lapidary work, including cabochons, carvings, and as decorative pocket stones or metaphysical tools.
Geological facts
Green common opal is often called 'Prase Opal' when colored by nickel inclusions. It contains between 3% and 21% water by weight and can become brittle if it dehydrates over time.
Field identification & locations
Identify by its characteristic waxy luster and smooth, shell-like conchoidal fracture. It is common in Madagascar, Tanzania, and Australia. Collectors should look for uniform color and lack of transparency.
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