sedimentary
Chert (varietal: Flint or River Pebble)
Silicon Dioxide (SiO₂)
Hardness: 7 (Mohs scale); Color: Dark brown, black, or grey; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Dark brown, black, or grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark brown, black, or grey; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or the chemical replacement of limestone/chalk by silica-rich groundwater. Often found as water-worn pebbles in riverbeds or glacial deposits.
Uses & applications
Historically used for stone tools (arrowheads) and fire-starting. Currently used in road construction as aggregate, decorative landscaping, and occasionally as semi-precious tumbled stones.
Geological facts
Because it breaks with a conchoidal (shell-like) fracture, it produces extremely sharp edges. It was the primary material for the Neolithic 'Stone Age' industry. When struck against steel, it creates sparks, which led to its use in flintlock firearms.
Field identification & locations
Identify in the field by its hardness (it will scratch glass and steel), its smooth water-worn texture, and characteristic curved 'chip' marks (conchoidal fractures). Common in river basins, coastal areas, and limestone-heavy regions.
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