sedimentary
Chert (specifically Flint or Dark Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale. Color: Dark brown to black. Luster: Dull to waxy. Crystal Structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
- Hardness
- 6
- Color
- Dark brown to black
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale. Color: Dark brown to black. Luster: Dull to waxy. Crystal Structure: Trigonal (microscopic). Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary rock layers, typically limestone or chalk.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Industrially used as an abrasive or in construction aggregate. Highly valued by flint-knappers for making arrowheads.
Geological facts
Chert was one of the most important materials for early humans because of its conchoidal fracture, which produces edges sharper than a steel scalpel. Darker varieties often get their color from organic carbon impurities.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture patterns and the fact it can scratch glass. It is often found as nodules in limestone or as stream-worn pebbles.
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