sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, or yellowish-brown, Luster: waxy to dull, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, or yellowish-brown, Luster: waxy to dull, Structure: cryptocrystalline, Cleavage: none (conchoidal fracture)
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in marine sedimentary environments. Found in Paleozoic to Cenozoic strata.
Uses & applications
Historically used for stone tools and starting fires. Currently used as road aggregate, architectural stone, and for lapidary work (polishing).
Geological facts
Chert has been used by humans for hundreds of thousands of years to make arrowheads and knives due to its sharp edges when broken.
Field identification & locations
Look for a very hard, smooth-textured rock that exhibits shell-like (conchoidal) fractures. Often found as nodules within limestone or as river pebbles.
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