sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

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

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

Identified May 24, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

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

Formation & geological history

Formed through the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or inorganic precipitation from silica-rich fluids in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons (arrowheads); currently used in road construction, as flint for fire-starting, and occasionally as an abrasive or in decorative jewelry.

Geological facts

Chert has been used by humans for over two million years to make sharp-edged tools due to its conchoidal fracture pattern, which creates razor-sharp edges.

Field identification & locations

Identify in the field by its waxy luster, extreme hardness (it will scratch glass), and the way it breaks into sharp, curved flakes. Commonly found in limestone outcrops or as rounded pebbles in stream beds.