sedimentary

Chert / Flint

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs. Color: Gray, tan, white, or brown. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.

Hardness: 6
Color: Gray, tan, white, or brown
Luster: Waxy to dull

Identified July 9, 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: 6.5-7 Mohs. Color: Gray, tan, white, or brown. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.

Formation & geological history

Formed by the precipitation of microcrystalline silica from water, often replacing organic material or occupying voids in limestone or chalk. Dates back to various geological periods from the Precambrian to present day.

Uses & applications

Used historically for stone tools and starting fires (flint). Currently used in road construction, landscape gravel, and occasionally as decorative stones.

Geological facts

Chert was one of the most important minerals for early humans because of its ability to break into sharp edges for arrowheads and scrapers. It is chemically identical to quartz but with a microscopic grain structure.

Field identification & locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and shell-like (conchoidal) fractures. Found worldwide in riverbeds, glacial tills, and sedimentary outcrops.