sedimentary
Chert (specifically Flint or Gray Chert)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, or white with waxy/dull luster; Crystal Structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, or white with waxy/dull luster
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, or white with waxy/dull luster; Crystal Structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Most specimens date from the Phanerozoic eon.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to sharp edges; currently used as road aggregate, in glass manufacturing, and as a component in ceramics.
Geological facts
Chert produces sparks when struck against steel, leading to its historical use in flintlock firearms. It is one of the most durable sedimentary rocks, often remaining long after the surrounding limestone has weathered away.
Field identification & locations
Identify in the field by its conchoidal (shell-like) fracture, sharp edges, and inability to be scratched by a steel knife. Common in limestone outcrops, riverbeds, and glacial till throughout North America and Europe.
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