sedimentary
Chert (and Jasper/Flint variety)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark brown, black, and tan mottled, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark brown, black, and tan mottled, Luster: Waxy to vitreous, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from silica-rich fluids replacing limestone or chalk. Commonly found in sedimentary basins.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for decorative stones, landscaping, and lapidary work.
Geological facts
Chert is so hard it can create sparks when struck against steel, which made its 'flint' variety essential for early firearms and fire-starting. It can preserve tiny fossils from millions of years ago.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture pattern and its ability to scratch glass. Commonly found in nodules within limestone beds or as rounded river cobbles.
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