sedimentary
Chert (Flint Gravel)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Dull gray-brown, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: Dull gray-brown, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
Formation & geological history
Formed through the precipitation of silica from groundwater or as nodules in limestone/chalk beds; often found as leftovers in river gravels or glacial till.
Uses & applications
Used in construction as road metal or concrete aggregate; historically used for stone tools and starting fires (striking steel).
Geological facts
Chert is extremely durable and chemically resistant to weathering, which is why small fragments like this often survive long after the surrounding rock has washed away.
Field identification & locations
Identify by its hardness (scratches glass) and its curved, glass-like fracture surfaces. Frequently found in driveway gravel or riverbeds.
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Brecciated Chert (Microcrystalline Silica)
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