sedimentary
Chert (variations include Flint or Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7; Color: Grey, tan, or brown; Luster: Waxy to dull; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7
- Color
- Grey, tan, or brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7; Color: Grey, tan, or brown; Luster: Waxy to dull; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or by chemical precipitation in groundwater replacing limestone/chalk. Often Precambrian to Cenozoic in age.
Uses & applications
Historically used for stone tools/weapons (knapping) and fire-starting. Currently used in road construction, as an abrasive, and sometimes in jewelry (as Jasper/Agate).
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it produces sparks due to the friction heating tiny particles of metal, making it essential for early survival and flintlock firearms.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and sharp edges. It is ubiquitous in limestone regions and gravel deposits globally.
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