sedimentary
Chert (Flint Pebble)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: tan, brownish-grey, Luster: dull to waxy, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, brownish-grey, Luster: dull to waxy, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture)
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments; age varies from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools and starting fires; currently used in construction aggregate and as a semi-precious stone for tumbling or lapidary work.
Geological facts
Chert is so hard it can scratch steel and was a primary material for prehistoric arrowheads and spear points because it breaks with sharp, glass-like edges.
Field identification & locations
Identify by its hardness (cannot be scratched by a knife), smooth conchoidal fracture patterns, and fine-grained texture; commonly found in limestone or chalk beds and river gravels.
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