sedimentary
Chert (Flint variety)
Microcrystalline context of Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: dull gray to dark brown with white rind, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: dull gray to dark brown with white rind, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments; typically found as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools and weapons (arrowheads); currently used in road construction, as decorative landscaping gravel, and as a sparking agent in fire-striking flint.
Geological facts
Chert has an extremely sharp edge when broken due to its conchoidal fracture pattern. It was one of the most important minerals for early humans for tool development.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its shell-like (conchoidal) curved breaks, and it often has a lighter colored 'rind' on the exterior. Common in riverbeds and limestone outcrops.
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