sedimentary
Chert (and associated Flint)
Silicon Dioxide (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6
Formation & geological history
Biogenic or chemical precipitation in marine environments; formed from the accumulation of silica-rich skeletal remains (radiolarians/diatoms) or through replacement of limestone/chalk by silica-rich fluids. Ages vary from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp edges; currently used as construction aggregate, road base, and for some decorative landscape gravel.
Geological facts
Darker varieties are specifically called flint. Because of its extreme hardness and way of breaking (conchoidal fracture), it was the primary material for the Stone Age tool industry and was later used to create sparks in flintlock firearms.
Field identification & locations
Identify by its waxy texture, inability to be scratched by steel, and 'scalloped' or shell-like chips along sharp edges. Common worldwide in limestone beds or as nodules in riverbeds.
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