sedimentary
Chert (Flint)
Chert (SiO2)
Hardness: 6.5-7 Mohs; Color: Gray, black, or brown; Luster: Waxy or dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.
- Hardness
- 6
- Color
- Gray, black, or brown
- Luster
- Waxy or dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Gray, black, or brown; Luster: Waxy or dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like sponges or radiolarians) or chemical precipitation in sedimentary basins; primarily found in limestone or chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads, knives); currently used in road construction, and occasionally in jewelry as cabochons.
Geological facts
Flint was a vital resource for prehistoric humans because its conchoidal fracture produces extremely sharp edges. It can also produce sparks when struck against steel.
Field identification & locations
Look for a microcrystalline texture, conchoidal (shell-like) fractures, and a hardness that can scratch glass. Common in chalk formations or coastal gravels.
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