sedimentary
Chert (often called Flint in dark varieties)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs/scratches glass); Color: Tan, brown, gray, earthy; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs/scratches glass)
- Color
- Tan, brown, gray, earthy
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs/scratches glass); Color: Tan, brown, gray, earthy; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica-rich fluids (often from marine organisms like diatoms or radiolarians) within sedimentary beds of limestone or chalk. Can occur in nodules or as primary beds.
Uses & applications
Historically used for making stone tools (arrowheads, knives) due to its sharp edges when broken. Historically used as a fire-starter. Modernly used in construction aggregate and occasionally as a gemstone (Jasper).
Geological facts
Chert was one of the most important minerals for early humans, providing the primary material for tools throughout the Stone Age. It is chemically identical to quartz but contains impurities and has an microscopic crystal structure.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel nail), its smooth conchoidal (shell-like) fracture, and its lack of obvious crystalline grains. Commonly found in riverbeds or weathered out of limestone cliffs.
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