sedimentary
Black Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to black, 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: Dark grey to black, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed primarily through the chemical precipitation of silica-rich fluids in marine sedimentary environments where it replaces carbonate material or accumulates as radiolarian/diatomaceous ooze.
Uses & applications
Historically used for stone tools and weapons (flintknapping); currently used in road construction as aggregate and occasionally as a decorative landscape stone.
Geological facts
Black chert is often called 'flint' and was prized by prehistoric humans because its conchoidal fracture creates edges sharper than a steel scalpel.
Field identification & locations
Identify in the field by its hardness (will scratch glass), smooth waxy feel, and curved 'shell-like' fracture patterns. Common in limestone and dolostone formations worldwide.
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