sedimentary
Chert (Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, brown, translucent to opaque, Luster: waxy to vitreous, Structure: microcrystalline quartz (trigonal), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, brown, translucent to opaque, Luster: waxy to vitreous, Structure: microcrystalline quartz (trigonal), Cleavage: none (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed in sedimentary environments via the concentration of silica, often originating from the skeletal remains of silica-secreting organisms like diatoms or radiolarians, or through chemical precipitation in groundwater replacing limestone.
Uses & applications
Historically used for stone tools (knapping) and fire-starting. Currently used in road surfacing, abrasives, and as a semi-precious gemstone in lapidary work.
Geological facts
Chert was one of the most vital materials for early humans because its conchoidal fracture creates edges sharper than a steel scalpel. High-quality varieties like Flint were essential for flintlock firearms.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (it will scratch glass and steel), and smooth, shell-like (conchoidal) fracture surfaces. Commonly found as nodules in limestone or as river gravel.
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