sedimentary

Chert (often called Flint)

Microcrystalline or cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Gray, tan, white, or brown; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, tan, white, or brown
Luster: Dull to waxy

Identified June 19, 2026More sedimentary →

Explore Chert (often called Flint) in the encyclopedia →

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: Gray, tan, white, or brown; Luster: Dull to waxy; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed as nodules or layers in limestone through the accumulation of silica-rich organic remains (like radiolarians or sponge spicules) or by secondary chemical replacement of host carbonates during diagenesis.

Uses & applications

Historically used for stone tools (knapping) and starting fires; currently used as road construction aggregate, gemstone cabochons, and as a refractory material.

Geological facts

Flint is a variety of chert that occurs specifically in chalk or marl. It was one of the first materials traded over long distances by prehistoric humans due to its ability to hold a sharp edge.

Field identification & locations

Identify in the field by its characteristic shell-like (conchoidal) fracture pattern and sharp edges; it will scratch glass easily. Commonly found in river beds, chalk cliffs, and limestone quarries.