sedimentary

Flint

SiO2 (Cryptocrystalline Quartz)

Hardness: 7 on the Mohs scale. Color: Dark gray, black, brown, or dark green with a lighter weathering cortex. Luster: Vitreous to waxy. Crystal structure: Microcrystalline/cryptocrystalline (quartz). Cleavage: None, exhibits distinct conchoidal fracture. Specific gravity: 2.60 - 2.65.

Hardness: 7 on the Mohs scale
Luster: Vitreous to waxy

Identified August 3, 2026More sedimentary →

Explore Flint 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 through the chemical replacement of calcium carbonate in chalk or limestone beds by silica-rich solutions derived from sponge spicules or dissolved silica in marine environments, typically during the Cretaceous period.

Uses & applications

Historically used for making stone tools, arrowheads, and striking fire with steel. Modern uses include abrasives, road aggregate, and historically in flintlock firearms.

Geological facts

Flint and chert are essentially the same material, though 'flint' typically refers specifically to the dark varieties found in chalk formations. Because of its conchoidal fracture, it breaks into extremely sharp edges, making it one of the most important materials for early human technology.

Field identification & locations

Easily identified by its dark, glassy appearance, conchoidal fracture surfaces, and often a porous white or light-colored outer rind (cortex). Commonly found embedded in chalk cliffs or as beach pebbles in regions with Cretaceous chalk deposits, such as southeastern England and parts of northern Europe.