sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Grey, tan, or brown; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Grey, tan, or brown
Luster: Waxy to dull

Identified May 28, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Grey, tan, or brown; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed from the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater. Found in sedimentary basins.

Uses & applications

Historically used for making stone tools and starting fires. Today used as an abrasive, in road construction, and as a collector specimen.

Geological facts

Chert is so hard it can scratch steel. When struck against steel, it produces sparks, which led to its widespread historical use in flintlock firearms and survival fire-starting.

Field identification & locations

Identify by its extremely smooth, waxy surface and the way it breaks into sharp, curved edges (conchoidal fracture). Commonly found as nodules in limestone or chalk beds.