sedimentary

Chert

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, and earthy brown; Luster: Dull to waxy; Texture: Fine-grained, microcrystalline; Fracture: conchoidal (shell-like)

Hardness: 7 on Mohs scale
Color: Gray, tan, and earthy brown
Luster: Dull to waxy

Identified July 4, 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: 7 on Mohs scale; Color: Gray, tan, and earthy brown; Luster: Dull to waxy; Texture: Fine-grained, microcrystalline; Fracture: conchoidal (shell-like)

Formation & geological history

Formed in marine environments through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when broken. Today used in construction aggregate and as a collector's specimen.

Geological facts

Chert is so hard it can scratch glass. It is often found as nodules or 'flint' in chalk and limestone formations. It was one of the first materials used by early humans for tool development.

Field identification & locations

Identified in the field by its extreme hardness, smooth conchoidal fracture, and resistance to chemical weathering. Commonly found in riverbeds or eroding out of sedimentary rock layers.