sedimentary
Chert (variations include Flint or Jasper)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, or tan; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, or tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, or tan; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of limestone/chalk by silica-rich fluids. Common in Precambrian to Cenozoic strata.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when broken. Current uses include road surfacing aggregate, landscaping, and as a component in glass and ceramics.
Geological facts
Chert is so hard it can strike sparks against steel, which led to its use in 'flintlock' firearms. It is incredibly durable and resistant to chemical weathering, often remaining as gravel long after surrounding rocks have eroded.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass easily. Commonly found in riverbeds, glacial tills, or embedded as nodules within limestone outcrops.
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