sedimentary
Chert (Common Opal or Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: White to off-white with waxy luster; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- White to off-white with waxy luster
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: White to off-white with waxy luster; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.
Formation & geological history
Formed via the accumulation of silica-rich fluids in cavities within sedimentary rocks or volcanic ash, occurring across various geological periods from Precambrian to recent times.
Uses & applications
Historically used for making sharp tools and arrowheads; currently used in jewelry (as cabochons), landscaping stones, and as a source of silica in glass manufacturing.
Geological facts
This specimen shows the characteristic waxy luster and conchoidal (shell-like) fracture typical of microcrystalline silica. It is often found as nodules within limestone or chalk.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy feel, and sharp, curved edges when broken. Found globally in sedimentary basins.
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