sedimentary

Chert (varietal Flint)

Silicon Dioxide (SiO2)

Hardness: 7 Mohs; Color: Tan to brown core with orange-red iron staining on the rind; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 Mohs
Color: Tan to brown core with orange-red iron staining on the rind
Luster: Dull to waxy

Identified February 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from groundwater filling pores or replacing minerals in limestone or chalk beds. Often dated from the Paleozoic to Mesozoic eras, appearing in nodules or bedded layers.

Uses & applications

Historically used for making stone tools and fire starters; currently used as road aggregate, in glass making, and occasionally as polished decorative stones or jewelry (lapidary).

Geological facts

Because of its extreme hardness and ability to fracture into razor-sharp edges, it was the primary material for spearheads and arrowheads in prehistoric cultures. Its name comes from the Greek word 'khertos' meaning 'fenced' or 'fortified'.

Field identification & locations

Identify by its waxy interior exposed by 'chips' (conchoidal fractures) and its ability to scratch glass. Commonly found in riverbeds or within limestone outcrops across the American Midwest and Western Europe.

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