sedimentary

Flint (Chert)

Cryptocrystalline Silica (SiO2)

Hardness: 7 on Mohs scale; Color: Smoky brown, black, or dark grey; Luster: Waxy or vitreous; structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Smoky brown, black, or dark grey
Luster: Waxy or vitreous

Identified June 7, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 on Mohs scale; Color: Smoky brown, black, or dark grey; Luster: Waxy or vitreous; structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed as nodules or masses in sedimentary rocks such as chalk or limestone. It results from the replacement of calcium carbonate by silica precipitated from groundwater, often during the Cretaceous period.

Uses & applications

Historically used for stone tools and weapons (arrowheads, knives); currently used in the manufacture of flint-lock mechanisms, as grinding media in ball mills, and for decorative landscaping.

Geological facts

Flint was the most important material for early human technological progress. When struck against steel or iron pyrites, it produces sparks, making it a primary fire-starting tool for millennia.

Field identification & locations

Identify by its smooth, waxy surface and sharp, curved (conchoidal) edges when broken. Commonly found in chalk deposits in the UK, France, and parts of the United States.