sedimentary
Chert (White Chert/Flint)
Chert (Microcrystalline or Cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: typically white, grey, or tan; Luster: waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- typically white, grey, or tan
- Luster
- waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: typically white, grey, or tan; Luster: waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of siliceous remains (like diatoms or radiolaria) or via chemical precipitation in ocean water. It can form as nodules in limestone or as bedded deposits. Geological age varies from Precambrian to Cenozoic.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to its sharp conchoidal edges. Currently used in road construction, as an abrasive, and occasionally as a gemstone in the form of jasper or agate.
Geological facts
Chert is so hard it can strike sparks against steel, which is why darker varieties (flint) were used in flintlock firearms. It is chemically identical to quartz but has a different internal microscopic texture.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in sedimentary basins, riverbeds, and as garden landscaping rock.
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