sedimentary

Chert (Flint)

Silicon dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: brown, gray, or tan; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: brown, gray, or tan
Luster: waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: brown, gray, or tan; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed from the precipitation of silica-rich fluids within sedimentary rock layers, often as nodules or beds in limestone or chalk through a process called diagenesis.

Uses & applications

Historically used for making tools and weapons; currently used in construction as aggregate, in ball mills, and by hobbyist collectors for knapping.

Geological facts

Flint is a variety of chert that has been used by humans for millions of years to start fires and create sharp edges for hunting and skinning.

Field identification & locations

Identify by its characteristic scalloped (conchoidal) fractures and sharp edges when broken. It cannot be scratched by a steel knife. Commonly found in riverbeds and chalk deposits.