sedimentary
Chert (Flint)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Gray to blackish-gray, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Gray to blackish-gray, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7
Formation & geological history
Formed primarily in marine environments where microcrystalline silica replaces organic matter or precipitates from silica-rich groundwater. It is often found in nodules within limestone or chalk beds.
Uses & applications
Historically used for making stone tools and starting fires. Modernly used as a road aggregate, in jewelry (when decorative), and as flint-striking steel for survivalist tools.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create sharp-edged tools like arrowheads and knives because of its predictable conchoidal fracture.
Field identification & locations
Identify by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is common in riverbeds and limestone outcrops.
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