sedimentary
Chert (Chert Nodule in Limestone Host)
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: Brown to tan waxy core with gray-white chalky exterior, Luster: Waxy to dull, Crystal structure: Microcrystalline/Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown to tan waxy core with gray-white chalky exterior, Luster: Waxy to dull, Crystal structure: Microcrystalline/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. Commonly found in Paleozoic and Mesozoic marine deposits where silica from sponges or diatoms concentrated in nodules.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracture; currently used for road ballast, decorative gravel, and lapidary specimens.
Geological facts
Chert and its variety flint were critical to human evolution, providing the sharp edges needed for survival. It is so tough that it often remains behind as 'lag' after the softer surrounding limestone erodes away.
Field identification & locations
Identify in the field by its waxy luster and smooth, shell-like (conchoidal) breaks that contrast with the gritty or dull texture of the surrounding sedimentary rock. Often found in riverbeds or outcrops of limestone.
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