sedimentary

Chert pebble

Microcrystalline silica (SiO2)

Hardness: 7 (Mohs scale), Color: Gray, brown, black, and tan, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65

Identified June 22, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: Gray, brown, black, and tan, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.65

Formation & geological history

Formed primarily as chemical precipitates or through the replacement of limestone by silica-rich fluids. Often contains microfossils and is found in marine sedimentary layers. Age varies by location, often millions of years old.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to conchoidal fracture. Currently used in construction aggregate, road base, and as decorative river stones.

Geological facts

Chert is essentially the same material as flint, though the name flint is usually reserved for nodules found in chalk. It was one of the first materials used by early humans for fire-starting and tool-making.

Field identification & locations

Identify by its hardness (scratches glass), waxy appearance, and smooth, curved (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial tills, and limestone outcrops.