sedimentary
Chert (Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or light gray; Luster: Dull/Waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or light gray
- Luster
- Dull/Waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or light gray; Luster: Dull/Waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7.
Formation & geological history
Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or sponges) within sedimentary basins. It can occur as nodules or bedded layers in limestone and chalk.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp conchoidal edges; currently used for road ballast, concrete aggregate, and occasionally in jewelry as cabochons or for fire-starting tools.
Geological facts
Chert was one of the most important minerals for early humans because its predictable fracture pattern allowed for the crafting of sharp weapons and tools. When struck against steel, it produces sparks used to ignite fire.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone outcrops or as river gravel.
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