sedimentary

Chert (specifically Flint)

Silicon dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Dark grey to brown with a light tan/white outer cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.6 – 2.7

Hardness: 7 (Mohs scale)
Color: Dark grey to brown with a light tan/white outer cortex
Luster: Waxy to dull

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as nodules or masses in sedimentary rocks like limestone or chalk. It is created through biochemical precipitation of silica from the remains of diatoms or sponges, often dating from the Cretaceous period to the Paleozoic era.

Uses & applications

Historically used for making stone tools (arrowheads, knives) and for starting fires (striking steel). Industrially used in road construction, ceramics, and occasionally as a gemstone in tumbled jewelry.

Geological facts

Flint was the primary material for the 'Stone Age' due to its ability to flake into razor-sharp edges. It is also the specific variety of chert used commonly in flintlock firearms to ignite gunpowder.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It often features a white, chalky outer 'skin' or cortex. Common in the UK, France, and the Midwestern United States.