sedimentary
Chert (Flint Nodules)
Sedimentary cryptocrystalline quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Dark brown, black, or grey; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Dark brown, black, or grey
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark brown, black, or grey; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through a chemical or biological process where silica precipitates out of groundwater or is concentrated from the remains of silica-shelled organisms in limestone or chalk beds. Often Cretaceous or Jurassic in age.
Uses & applications
Historically used for making stone tools (arrowheads, knives) due to its sharp conchoidal fracture. Used modernly as a decorative landscape stone and in some road construction.
Geological facts
Flint was one of the most important minerals for early humans, used both for tools and as a fire-starter because it creates sparks when struck against steel. The dark color often comes from trapped organic matter.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) breaking pattern and its ability to scratch glass. It often features a white outer 'cortex' or weathered layer when found in chalk deposits.
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