sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, brown, or tan with a white cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, brown, or tan with a white cortex
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, brown, or tan with a white cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation within limestone or chalk beds. Commonly found in Mesozoic or Paleozoic sedimentary layers.

Uses & applications

Historically used for stone tools (arrowheads, knives) and strike-a-lights for fire starting. Modernly used as a road aggregate, in glass making, and as decorative gravel.

Geological facts

Flint is a variety of chert that occurs primarily in chalk or marly limestone. It was one of the most important minerals for early humans because of its predictable conchoidal fracture which allows for sharp edges.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture and its ability to scratch glass. It often features a chalky white outer layer (cortex). Commonly found in riverbeds or within limestone outcrops.