sedimentary
Chert (varieties include Flint and Jasper)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, brown, or white (based on impurities); Luster: Waxy to dull; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, brown, or white (based on impurities)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, brown, or white (based on impurities); Luster: Waxy to dull; Structure: Cryptocrystalline/Microcrystalline; 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. It occurs as nodules or layered beds in sedimentary sequences ranging from Precambrian to Cenozoic ages.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp edges; currently used as a raw material for road construction, grindstones, and sometimes as a gemstone (Jasper varieties).
Geological facts
Chert was one of the first materials used by early humans to create fire because it produces sparks when struck against iron or steel. Its conchoidal fracture allows it to be chipped into incredibly sharp, surgical-grade edges.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its glass-like, shell-shaped fracture surfaces (conchoidal). Often found as nodules protruding from weathered limestone outcrops or as river-worn cobbles.
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