sedimentary
Chert (Flint Nodules)
Chert (Silicon Dioxide, SiO2)
Hardness: 7 (Mohs scale); Color: Gray to black interior with a white/tan chalky cortex; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Gray to black interior with a white/tan chalky cortex
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray to black interior with a white/tan chalky cortex; Luster: Waxy to vitreous; Crystal Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed by the precipitation of silica in sedimentary rock layers, often as nodules within limestone or chalk. Many date back to the Cretaceous period as replacements for organic material or filling cavities.
Uses & applications
Historically used for stone tools and weapons (arrowheads). Modernly used as road gravel, in concrete aggregate, and for lapidary work/jewelry if color-patterned.
Geological facts
Flint was one of the most important minerals for early human survival because it breaks with sharp edges and creates sparks when struck against iron or pyrite to start fires.
Field identification & locations
Identify by its 'cortex' (rough outer skin) and glass-like, sharp edges when broken. It is extremely hard and cannot be scratched by a steel nail. Commonly found in chalk cliffs and riverbeds.
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