sedimentary
Chert / Flint
Silicon dioxide (SiO2)
Hardness: 7 (Mohs scale). Color: Tan, cream, and grey. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, cream, and grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale). Color: Tan, cream, and grey. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich microfossils or precipitation from silica-saturated water in marine or groundwater environments. Can occur as nodules in limestone or chalk beds.
Uses & applications
Historically used for stone tools (knapping) and sparking fire. Today used for decorative landscaping, road gravel, and as a raw material for ceramics and jewelry (agate varieties).
Geological facts
Flint is a variety of chert that was essential to prehistoric humans for crafting arrowheads and axes because it fractures predictably into sharp edges. It can strike sparks against steel to ignite gunpowder or tinder.
Field identification & locations
Identify in the field by its very hard surface (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breakage patterns. Common in chalk districts and riverbeds.
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