sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs; Color: Brown, tan, and dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65

Hardness: 6
Color: Brown, tan, and dark gray
Luster: Waxy to dull

Identified June 5, 2026More sedimentary →

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: Brown, tan, and dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65

Formation & geological history

Forms from the accumulation of siliceous remains of organisms or chemical precipitation in marine environments, often within limestone or chalk beds. Commonly associated with Cretaceous period formations.

Uses & applications

Historically used for stone tools and weapons (knapping); modern uses include construction aggregate, road base, and decorative landscaping.

Geological facts

Flint is a variety of chert that was essential to human survival for thousands of years because it produces a sharp edge and sparks when struck with steel.

Field identification & locations

Identify by its very smooth, seashell-like (conchoidal) fracture pattern and its ability to scratch glass. Often found as nodules or pebbles in riverbeds and coastal regions.