sedimentary
Chert (often called Flint when dark)
Sedimentary cryptocrystalline quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brown/Tan (various), Luster: Waxy to dull, Crystal structure: Microcrystalline/Trigonal, Cleavage: None (conchoidal fracture), Specific gravity: 2.6
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown/Tan (various), Luster: Waxy to dull, Crystal structure: Microcrystalline/Trigonal, Cleavage: None (conchoidal fracture), Specific gravity: 2.6
Formation & geological history
Formed from the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within sedimentary host rocks like limestone.
Uses & applications
Historically used for stone tools (arrowheads). Today used in road construction, as decorative gravel, and occasionally as a low-cost gemstone for tumbling.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges sharper than a steel scalpel. Darker, high-quality varieties are known as flint and were essential for starting fires and discharging early firearms.
Field identification & locations
Identify in the field by its waxy appearance and 'shell-like' curved fractures. It will easily scratch glass and steel. Look for it as nodules in limestone or as water-worn pebbles in riverbeds.
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