sedimentary
Chert (containing Quartz/Chalcedony)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: White, tan, and brownish-yellow hues, Luster: Waxy to vitreous when wet, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: White, tan, and brownish-yellow hues, Luster: Waxy to vitreous when wet, Crystal Structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich microfossils or via chemical precipitation from silica-rich fluids within sedimentary rock layers like limestone or chalk. Commonly Paleozoic to Mesozoic in age.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for road gravel, construction fill, and lapidary tumbling for hobbyists.
Geological facts
Chert is so hard it can scratch glass and steel. It was one of the most vital materials for early humans because its conchoidal fracture creates edges sharper than a surgical scalpel.
Field identification & locations
Identify by its extreme hardness and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial till, or embedded in limestone outcrops.
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