mineraloid
Common Opal
Hydrated Amorphous Silica (SiO2·nH2O)
Hardness: 5.5-6.0 on Mohs scale; Color: Pale green, white, or yellowish; Luster: Waxy to vitreous; Crystal structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.
- Hardness
- 5
- Color
- Pale green, white, or yellowish
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 5.5-6.0 on Mohs scale; Color: Pale green, white, or yellowish; Luster: Waxy to vitreous; Crystal structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.
Formation & geological history
Formed from silica-rich groundwater that precipitates in the cavities and fissures of igneous or sedimentary rocks. It is most commonly found in volcanic host rocks where hydrothermal activity has occurred.
Uses & applications
Used primarily for lapidary work, cabochons for jewelry, and as decorative pocket stones. Unlike precious opal, it lacks 'play-of-color' but is valued for its waxy translucency.
Geological facts
Common opal is technically a mineraloid rather than a mineral because it lacks a crystalline structure. It contains between 3% and 21% water by weight within its structure.
Field identification & locations
Identify in the field by its waxy texture, lack of cleavage, and conchoidal (shell-like) fracture patterns. Frequently found in regions with ancient volcanic activity like the Western United States, Australia, and Mexico.
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