sedimentary
Chert (Black Chert or Flint)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Dark grey to black
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica from water, often in marine environments where microfossils like diatoms or radiolarians accumulate. It can be found in a variety of geological ages from Precambrian to recent times.
Uses & applications
Historically used for making stone tools (knapping) due to its sharp edges; currently used as road aggregate, in glass making, or as gemstone/jewelry in polished form.
Geological facts
Black chert is often referred to as flint and was one of the most important minerals for early humans to create fire and weapons. Some cherts contain exceptionally preserved microfossils.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), its conchoidal (shell-like) fracture pattern, and its smooth, waxy texture when freshly broken. Common in sedimentary basins worldwide.
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