mineraloid
Common Opal in Host Rock
Hydrated Silica (SiO2·nH2O)
Hardness: 5.5-6.5 on Mohs scale; Color: Milky white to pale green or colorless; Luster: Vitreous to waxy; Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.
- Hardness
- 5
- Color
- Milky white to pale green or colorless
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 5.5-6.5 on Mohs scale; Color: Milky white to pale green or colorless; Luster: Vitreous to waxy; Crystal Structure: Amorphous (non-crystalline); Specific Gravity: 1.9-2.3.
Formation & geological history
Formed from silica-rich water precipitating in the cavities or fissures of igneous or sedimentary rocks, often following volcanic activity. This specimen appears to be filling a vesicle within a basaltic or volcanic matrix.
Uses & applications
Common opal is used for lapidary work like cabochons, metaphysical collecting, and as a raw mineral specimen for educational purposes. Unlike 'precious opal,' it lacks play-of-color.
Geological facts
Opal is a mineraloid because it lacks a crystalline structure. It can contain up to 21% water within its silica structure, though it is usually between 6% and 10%.
Field identification & locations
Identify in the field by its waxy luster and conchoidal (shell-like) fracture. Look for it in volcanic regions or seams of sedimentary rock. It will not scratch glass as easily as quartz but is harder than calcite.
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