sedimentary
Chert (or Jasper)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Brownish-red to tan, Luster: Dull to waxy, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Brownish-red to tan, Luster: Dull to waxy, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich remains of microscopic organisms like diatoms or radiolarians, or through chemical precipitation in seafloor or groundwater environments.
Uses & applications
Historically used for stone tools due to sharp fracture edges; currently used for road gravel, drainage rocks, and occasionally lapidary work.
Geological facts
Chert is so hard it can strike sparks against steel, which led to its historic use in flintlock firearms. Jasper is specifically the opaque, iron-stained variety of chert.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds and as nodules in limestone.
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