sedimentary

Chert

Microcrystalline Quartz (SiO2)

Mohs hardness of 7; waxy to dull luster; conchoidal fracture; tan to light brown color; opaque to translucent edges; no cleavage.

Identified July 20, 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

Mohs hardness of 7; waxy to dull luster; conchoidal fracture; tan to light brown color; opaque to translucent edges; no cleavage.

Formation & geological history

Formed primarily as a biological or chemical precipitate in marine environments where silica-rich fluids replace calcium carbonate or accumulate as radiolarian/diatomaceous ooze.

Uses & applications

Historically used for stone tools and fire-starting; currently used for road surfacing, decorative stone, and as a component in some ceramics.

Geological facts

Chert was the primary material for prehistoric arrowheads and scrapers due to its extreme hardness and the sharpness of its conchoidal fracture edges.

Field identification & locations

Identify by its waxy texture, inability to be scratched by a steel nail, and shell-like fracture patterns. Often found as nodules within limestone or chalk beds.