sedimentary

Flint (Chert)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Brownish-orange weathered exterior (cortex) with a glassy blue-grey or dark grey interior; Luster: Waxy to vitreous; Cleavage: None (conchoidal fracture); Crystal Structure: Cryptocrystalline quartz.

Hardness: 7 (Mohs scale)
Luster: Waxy to vitreous

Identified July 14, 2026More sedimentary →

Explore Flint (Chert) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed as nodules within sedimentary rocks like chalk and limestone. Silica-rich fluids precipitate into cavities or replace organic matter. Common in the Cretaceous chalk of Southern England (approx. 66-100 million years old).

Uses & applications

Historically used for stone tools (knapping) and later for strike-a-lights in flintlock firearms. Currently used for gravel, road construction, and decorative walling (flint knapping) in English architecture.

Geological facts

This specimen shows a characteristic 'cortex' or weathered outer rind stained by iron oxides. The coordinates (51.8N, 1.1E) place this in Essex, UK, where flint is the most common native rock due to the underlying Cretaceous chalk and glacial drift deposits.

Field identification & locations

Identify by its extremely smooth, glass-like fracture (conchoidal) and its ability to scratch glass. It is often found in riverbeds, fields, and coastal beaches throughout East Anglia and the Thames Estuary.