sedimentary
Chert (Cherty Shale / Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or black; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, white, or black
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, white, or black; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from groundwater in sedimentary environments, often replacing organic material or carbonate rocks. Many occur in limestone or dolostone modules and can date from the Precambrian to Cenozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to conchoidal fracturing. Modern uses include road surfacing material, concrete aggregate, and occasional use in jewelry (as jasper or flint).
Geological facts
Because of its extreme hardness and ability to hold a sharp edge, chert was one of the most vital survival materials for Early Man. It can create sparks when struck against steel, leading to its specialized use in flintlock firearms.
Field identification & locations
Identify by its waxy texture and shell-like (conchoidal) breaking pattern. It will easily scratch glass and cannot be scratched by a steel pocketknife. Found globally in limestone formations and riverbeds.
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