sedimentary
Chert (Flint) with Chalk Cortex
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark gray/brown interior with white outer cortex, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark gray/brown interior with white outer cortex, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed as nodules in limestone or chalk beds by the replacement of calcium carbonate with silica-rich fluids. Often found in Cretaceous period formations around 65-145 million years old.
Uses & applications
Historically used for stone tools and starting fires; currently used in grinding media, construction aggregate, and as a collector specimen.
Geological facts
Flint was the most essential material for prehistoric human survival, used to create arrowheads, scrapers, and knives due to its predictable conchoidal fracturing.
Field identification & locations
Identify by the sharp edges where broken and the presence of a white, chalky outer rind (cortex). Common in chalk downlands, riverbeds, and coastal beaches.
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