sedimentary
Chert
Microcrystalline silica (SiO2)
Hardness: 7 (Mohs scale), Color: Variegated levels of gray, brown, and tan often with iron staining, Luster: Waxy to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: Variegated levels of gray, brown, and tan often with iron staining, Luster: Waxy to dull, Crystal structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture)
Formation & geological history
Formed primarily as chemical precipitates from silica-rich groundwater or from the accumulation of siliceous remains of organisms like diatoms or radiolarians, typically in marine environments during various geological eras.
Uses & applications
Historically used for making sharp tools like arrowheads and scrapers. Modern uses include road surfacing, as an abrasive, and occasionally in jewelry as tumbled stones.
Geological facts
Chert is essentially the same material as flint, but the term 'flint' is usually reserved for specimens found in chalk or marly limestone. It is one of the most durable sedimentary rocks.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and its characteristic conchoidal (shell-like) fracture which creates very sharp edges. Commonly found as nodules or layers within limestone beds.
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