sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness of 7 on the Mohs scale, dull to waxy luster, conchoidal fracture, crypto-crystalline or microcrystalline structure, colors range widely from white to gray, brown, black, or red.

Identified August 6, 2026More sedimentary →

Explore Chert in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness of 7 on the Mohs scale, dull to waxy luster, conchoidal fracture, crypto-crystalline or microcrystalline structure, colors range widely from white to gray, brown, black, or red.

Formation & geological history

Formed via the chemical precipitation of silica from aqueous solutions, often replacing carbonate rocks (limestone) during diagenesis or accumulating from the skeletal remains of siliceous organisms like radiolarians and diatoms.

Uses & applications

Historically used for making stone tools (arrowheads, spear points, and blades) due to its sharp conchoidal fracture. Also used historically for fire-striking.

Geological facts

Chert that occurs in nodules in chalk or limestone formations is commonly referred to as flint. Because of its durability and sharp edges, it was one of the most vital materials for early human stone-tool technology.

Field identification & locations

Easily identified by its extreme hardness, waxy luster, and distinctive conchoidal fracture that creates razor-sharp edges when broken. Found worldwide in sedimentary sequences, particularly limestone and chalk beds.