sedimentary

Chert (and Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, black, white, or brown; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, black, white, or brown
Luster: Waxy to dull

Identified March 7, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, black, white, or brown; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica-rich fluids in sedimentary environments. Common as nodules in limestone or chalk beds, or as massive layered deposits. It can be biological (from radiolaria or diatoms) or chemical in origin.

Uses & applications

Historically used for making stone tools (arrowheads, axes) due to its sharp edges; used in industrial grinding, road construction, and as flint for fire-starting.

Geological facts

Chert was one of the most important minerals for early humans. Its ability to fracture into razor-sharp edges made it the primary material for the Stone Age tool industry. Flint is a specific variety of chert often associated with chalk.

Field identification & locations

Identify in the field by its distinct conchoidal fracture (curved, glass-like breaks) and extreme hardness (it will scratch steel). Commonly found in riverbeds, glacial till, and coastal chalk cliffs.