sedimentary
Chert
Opaline or Microcrystalline Jasper/Chert (SiO2)
Hardness: 6.5-7 (Mohs), Color: brownish-grey with waxy luster, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7. It is opaque to slightly translucent at the edges.
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs), Color: brownish-grey with waxy luster, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6-2.7. It is opaque to slightly translucent at the edges.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from silica-rich fluids within sedimentary basins. It can occur as nodules or bedded layers.
Uses & applications
Used historically for stone tools due to its sharp edges; modern uses include landscaping, construction aggregate, and occasionally as a base for lapidary work.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it was a primary material for prehistoric arrowheads and scrapers. It is often found as nodules inside limestone or chalk.
Field identification & locations
Identifiable by its smoothness, waxy luster, and sharp edges where broken. Can be found globally in sedimentary environments. Its hardness prevents it from being scratched by a steel knife.
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