sedimentary
Chert Nodule (Flint)
Cryptocrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Black/dark gray core with a white/tan chalky outer cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Black/dark gray core with a white/tan chalky outer cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Black/dark gray core with a white/tan chalky outer cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the replacement of limestone or chalk by silica-rich fluids during diagenesis. They often precipitate around organic nuclei within seabed sediments.
Uses & applications
Historically used for stone tools (knapping) and sparking fires. Modern uses include abrasives, road construction, and as a collector's curiosity due to the distinct 'eye' appearance.
Geological facts
Flint was the primary material for the Stone Age technology. The white outer layer, known as the cortex, is actually the same silica but with a more porous, weathered structure than the dense dark center.
Field identification & locations
Look for rounded, potato-like nodules in limestone or chalk cliffs. When broken, they reveal a sharp, glass-like interior. Commonly found in Dover (UK), Texas, and the Midwest USA.
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