sedimentary
Chert (Flint Nodule)
Microcrystalline quartz (SiO2)
Hardness: 7 Mohs; Color: Dark grey to black with white cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)
- Hardness
- 7 Mohs
- Color
- Dark grey to black with white cortex
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Dark grey to black with white cortex; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed as nodules within limestone or chalk beds as silica-rich fluids precipitate in cavities or replace organic material during diagenesis.
Uses & applications
Historically used for stone tools and fire-starting; modern use in construction aggregate and as a collector's specimen.
Geological facts
Flint was the primary material for prehistoric survival tools due to its ability to hold a razor-sharp edge. It produces sparks when struck against steel.
Field identification & locations
Identify by its waxy texture, extreme hardness (scratches glass), and shell-like (conchoidal) curved fracture surfaces. Often found in chalk cliffs.
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