sedimentary

Chert

Chert (Microcrystalline Silica, SiO2)

Hardness: 7 Mohs; Color: Greyish-white to tan; Luster: Dull to waxy; Texture: Fine-grained, cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 Mohs
Color: Greyish-white to tan
Luster: Dull to waxy

Identified July 9, 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 Mohs; Color: Greyish-white to tan; Luster: Dull to waxy; Texture: Fine-grained, cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & geological history

Formed primarily in deep marine environments through the accumulation of siliceous remains of organisms like diatoms and radiolarians, or via chemical precipitation from silica-rich fluids. Common in sedimentary layers from the Paleozoic to Cenozoic eras.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp edges; currently used for road ballast, grinding media, and sometimes as a minor decorative stone.

Geological facts

Because it breaks with a conchoidal fracture like glass, it was a preferred material for prehistoric humans to create sharp cutting tools. It is a very durable rock that often survives as pebbles in gravel beds long after the surrounding rock has weathered away.

Field identification & locations

Identify in the field by its hardness (will scratch glass), its waxy luster, and its characteristic curved, shell-like fracture patterns. Often found within limestone or chalk beds.