sedimentary

Chert (Flint)

Silicon dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, gray, or yellow; Luster: Waxy to dull; Crystal structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan, brown, gray, or yellow
Luster: Waxy to dull

Identified July 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by the chemical precipitation of silica in sedimentary basins. Most chert is Paleozoic or younger in age.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used as road aggregate, in glass manufacturing, and as a sparking tool in 'flint and steel' fire starters.

Geological facts

Because it breaks with a conchoidal fracture (like glass), it creates incredibly sharp edges. It was one of the first materials traded by prehistoric humans over long distances.

Field identification & locations

Identify in the field by its waxy texture, inability to be scratched by a steel knife, and characteristic shell-like (conchoidal) curved fractures. Common in limestone and chalk beds.