sedimentary
Chert (varieties include Flint and Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, black, or tan; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, black, or tan
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, black, or tan; Luster: Vitreous to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation of silica-rich groundwater within limestone or chalk beds. Most specimens date from the Paleozoic or Mesozoic eras.
Uses & applications
Used historically for stone tools due to its sharp conchoidal fracture. Modern uses include road surfacing (as gravel), decorative landscaping, and occasionally as semi-precious gemstones in jewelry (Jasper/Agate varieties).
Geological facts
Chert was one of the most vital materials for early humans because its sharp edges were used to make arrowheads, scrapers, and knives. It is also famous for its ability to create sparks when struck against steel, which is why darker varieties are often called 'flint'.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass easily. It is commonly found in nodules within limestone formations or as river-worn pebbles. It is ubiquitous in the American Midwest and the Chalk Downlands of England.
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