sedimentary

Chert (chalcedony)

Microcrystalline quartz, SiO₂

AI-generated identification · may be incorrect

The small brown-gray piece has a waxy luster and irregular, shell-like fracture, consistent with chert. Quartz hardness is about 7 Mohs; its crystals are too fine to see unaided, and it has no prominent cleavage.

Identified October 4, 2026More sedimentary →

Explore Chert (chalcedony) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Chert forms when silica-rich material precipitates or accumulates and becomes lithified, often in marine sediments; it occurs in rocks of many geological ages.

Uses & applications

Chert has been used for sharp-edged tools and is sometimes cut or polished for ornamental objects; most small rough fragments have little commercial value.

Geological facts

When struck, chert can break into sharp flakes with curved conchoidal surfaces, making it a widely used prehistoric toolstone.

Field identification & locations

Look for a hard, fine-grained surface, waxy appearance, and sharp conchoidal fracture; brown color alone is not diagnostic. Chert is widespread in sedimentary formations worldwide.