sedimentary
Chert (Black Chert/Flint)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Black, dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Black, dark gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Black, dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (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 by chemical precipitation from silica-rich fluids within limestone or chalk beds. Often Cretaceous or Paleozoic in age.
Uses & applications
Historically used for stone tools (arrowheads, knives) due to sharp edges; currently used as a gemstone (onyx-like appearance), in construction aggregate, or for fire-starting strikers.
Geological facts
Black chert is often specifically called flint. It was one of the most vital materials for prehistoric humans to create fire and weaponry. Some specimens contain microfossils visible only under magnification.
Field identification & locations
Identify by its characteristic seashell-like (conchoidal) fracture and its ability to scratch glass. Commonly found in riverbeds, glacial till, or embedded in limestone outcrops. Use a streak plate; it should leave no streak or a white one.
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