sedimentary
Flint (Chert)
Silicon dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 7 (Mohs scale); Color: Dark grey, black, or brown inside with a white weathered cortex; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark grey, black, or brown inside with a white weathered cortex; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed as nodules in sedimentary rocks like chalk or limestone by the replacement of calcium carbonate with silica-rich water. Common in Cretaceous age deposits.
Uses & applications
Historically used for stone tools and fire-starting; modern use includes road construction, glass manufacturing, and flintlock mechanisms for historical firearms.
Geological facts
Flint was the primary material for the Stone Age tool industry. When struck against steel, it creates a spark hot enough to ignite tinder, a property used for centuries in flintlock guns.
Field identification & locations
Identify by its waxy luster on a fresh break, conchoidal (shell-like) fracture patterns, and the white chalky outer layer known as the cortex. Often found on beaches or in chalk hills.
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