sedimentary
Black Chert
Carbonaceous Chert (Silicon dioxide, SiO2)
Hardness: 7 (Mohs scale), Color: Black to dark gray, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
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Physical properties
Hardness: 7 (Mohs scale), Color: Black to dark gray, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine environments; the dark color is primarily due to included organic matter. Found in rocks of various ages ranging from Precambrian to Cenozoic.
Uses & applications
Used historically for stone tools due to sharp edges; currently used as construction aggregate, for landscaping, and as a semi-precious stone in jewelry or flintknapping.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create arrowheads and scrapers. Black chert is often specifically called 'flint' if it occurs in chalk or marly limestone.
Field identification & locations
Identified in the field by its extreme hardness (cannot be scratched by steel), smooth conchoidal (shell-like) fracture surfaces, and waxy texture. Commonly found in limestone beds.
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