sedimentary
Chert (Flint with Chalk/Calcite)
Sedimentary cryptocrystalline quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Mix of gray and white, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (Conchoidal fracture), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Mix of gray and white, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (Conchoidal fracture), Specific gravity: 2.6-2.7
Formation & geological history
Formed in marine sedimentary environments where silica-rich fluids replaced pre-existing carbonate sediments (chalk or limestone). Common in Upper Cretaceous deposits.
Uses & applications
Historically used for making stone tools and fire-starting; currently used as construction aggregate, road metal, and in some lapidary hobbies.
Geological facts
Dark flint nodules are often found encased in a white 'cortex' of chalk or weathered limestone, creating the multi-colored appearance seen in this pebble.
Field identification & locations
Identify by its extreme hardness (scratches glass) and smooth, shell-like (conchoidal) fracture surfaces. Commonly found on beaches and in gravel pits near limestone regions.
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