sedimentary
Chert (Black Chert/Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to black, Luster: Waxy to dull, Crystal Structure: Microcrystalline/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: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments; typically found as nodules or bedded layers in limestone or chalk.
Uses & applications
Historically used for stone tools (axes, arrowheads) and fire-starting; currently used as a gemstone (lapidary), as a component in road construction, and for aquarium decoration.
Geological facts
Black chert is often colored by organic matter. Famous for its conchoidal (shell-like) fracture which creates razor-sharp edges, a property that defined the Stone Age technology for millions of years.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and curved, glass-like fracture surfaces. Found worldwide in sedimentary basins. Collectors look for uniform color and lack of internal fractures.
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