sedimentary
Chert (variations include Flint)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, black, brown, or tan; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, black, brown, or tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, black, brown, or tan; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within limestone or chalk beds. Often associated with Cretaceous and Jurassic geological periods.
Uses & applications
Used historically for stone tools (arrowheads, knives) due to its sharp edges. Modern uses include road surfacing, construction aggregates, and historically as the strike-element in flintlock firearms.
Geological facts
Chert was one of the most important minerals for early humans. When struck, it produces a sharp, glass-like edge and can create sparks when struck against steel, which made it vital for survival and tool-making for millennia.
Field identification & locations
Identify in the field by its very hard surface (it will scratch glass), its smooth, shell-like (conchoidal) fracture pattern, and often a white chalky outer 'rind' or cortex. Commonly found in nodular form within limestone outcrops.
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