sedimentary
Chert (with Limestone matrix)
Chert (Silica: SiO2)
Hardness: 7 (Mohs scale), Color: Dark grey to black nodule in a tan/grey matrix, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline (cannot see crystals with naked eye), Cleavage: None (conchoidal fracture).
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark grey to black nodule in a tan/grey matrix, Luster: Waxy to dull, Crystal Structure: Microcrystalline/Cryptocrystalline (cannot see crystals with naked eye), Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or sponges) or chemical precipitation in sedimentary basins, typically within marine limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads). Modern uses include road aggregate, gemstones (when colorful), and fire-starting (flint).
Geological facts
Dark-colored chert is often specifically called 'flint'. Because it breaks with a conchoidal fracture (smooth, curved surfaces), prehistoric humans prized it for creating sharp edges for hunting and cutting.
Field identification & locations
Identify in the field by its waxy luster and smooth, sharp edges when broken. It will scratch glass easily but will not be scratched by a steel knife. Commonly found in limestone outcrops.
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