sedimentary

Chert (Flint)

Microcrystalline or cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Gray, tan, brown, or black; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Gray, tan, brown, or black
Luster: Dull to waxy

Identified May 24, 2026More sedimentary →

Explore Chert (Flint) 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: Gray, tan, brown, or black; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); 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 through the chemical replacement of limestone/dolomite by silica-rich fluids. Found in marine sedimentary environments.

Uses & applications

Historically used for stone tools (arrowheads, knives) and strike-a-lights; currently used in construction aggregate and as a semi-precious gemstone in jewelry when colorful.

Geological facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's steel scalpel. It was one of the most vital materials for prehistoric human survival.

Field identification & locations

Identify in the field by its hardness (cannot be scratched by steel), waxy texture on fresh surfaces, and smooth, curved shell-like fracture patterns. Common in limestone outcrops.