sedimentary

Chert (Flint variety)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Black, dark brown, or grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Black, dark brown, or grey
Luster: Waxy to dull

Identified July 21, 2026More sedimentary →

Explore Chert (Flint variety) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 (Mohs scale); Color: Black, dark brown, or grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds during various geological periods.

Uses & applications

Historically used for stone tools and weapons (arrowheads); currently used for fire-striking, road surfacing, and as a component in certain ceramics and glass making.

Geological facts

Flint was one of the first materials used by early humans to create fire and sharp tools due to its predictable conchoidal fracture pattern and high hardness.

Field identification & locations

Identify in the field by its very smooth, glass-like fracture surfaces and ability to scratch glass. It often has a white, chalky outer coating called a cortex.