sedimentary
Chert
Microcrystalline or cryptocrystalline quartz (SiO2)
Hardness: 7 (Mohs), Color: Light gray to tan, Luster: Dull to waxy, Crystal structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
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Physical properties
Hardness: 7 (Mohs), Color: Light gray to tan, Luster: Dull to waxy, Crystal structure: Trigonal/Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65
Formation & geological history
Formed via the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in sedimentary basins. This specimen appears to be a weathered or partially knapped fragment.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; modernly used as road aggregate and in some primitive fire-starting kits.
Geological facts
Because it breaks with a conchoidal fracture like glass, it produces razor-sharp edges. It is often found as nodules inside limestone or chalk formations.
Field identification & locations
Identify by its hardness (cannot be scratched by steel), smooth conchoidal fracture patterns, and fine-grained texture. Commonly found in riverbeds or eroding out of limestone cliffs.
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