mineral
Common Opal / Chert
Hydrated silica (SiO2·nH2O) or Microcrystalline Quartz (SiO2)
Hardness: 5.5-6.5 (Mohs scale); Color: Tan, beige, or yellowish milky; Luster: Waxy to sub-vitreous; Structure: Amorphous (Opal) or Cryptocrystalline (Chert); Fracture: Conchoidal
- Hardness
- 5
- Color
- Tan, beige, or yellowish milky
- Luster
- Waxy to sub-vitreous
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Physical properties
Hardness: 5.5-6.5 (Mohs scale); Color: Tan, beige, or yellowish milky; Luster: Waxy to sub-vitreous; Structure: Amorphous (Opal) or Cryptocrystalline (Chert); Fracture: Conchoidal
Formation & geological history
Formed by the deposition of silica-rich fluids in volcanic cavities or sedimentary basins. Common in the Zagros fold belt region of Iran (Shiraz area) where sedimentary layers frequently contain silicate nodules.
Uses & applications
Used for carvings, as a source for tools in prehistoric times, and as lower-grade decorative stones or architectural fill.
Geological facts
The user mentioned 'اوپال' (Opal). While this specimen looks like a 'Common Opal' (potch) because it lacks 'play-of-color', it shares the same chemical composition as the precious varieties. It is often found as nodules within limestone or volcanic rock.
Field identification & locations
Identify by its smooth, waxy feel and shell-like (conchoidal) fracture when broken. It will scratch glass but cannot be scratched by a steel knife. Common in the sedimentary sequences around Shiraz.
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