sedimentary
Chert Nodule
Chert (Cryptocrystalline variety of Quartz, SiO2)
Hardness: 7 (Mohs scale), Color: Brown, gray, or tan with a weathered yellowish rind, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, gray, or tan with a weathered yellowish rind, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the chemical precipitation of silica-rich fluids within sedimentary host rocks like limestone or chalk. Often replaces organic matter or fills voids; ages range from Precambrian to Cenozoic.
Uses & applications
Used historically for stone tools (flintknapping), as road aggregate in construction, and in abrasive applications. Smooth nodules are popular for landscape decoration and rock tumbling.
Geological facts
Chert is so durable that it often remains as 'float' in soil long after its parent limestone has weathered away. Ancient civilizations used chert and its variety, flint, to create sharp-edged tools and to start fires.
Field identification & locations
Identify by its rounded or irregular 'knobby' shape and extreme hardness (it will scratch glass). Often found in fields, riverbeds, or embedded in limestone outcrops. Look for smooth, shell-like (conchoidal) fractures if chipped.
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