sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs), Color: Gray to tan with waxy luster, Texture: Cryptocrystalline, Fracture: Conchoidal (shell-like curves), Luster: Waxy to dull, Cleavage: None.

Luster: Waxy to dull, Cleavage: None

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs), Color: Gray to tan with waxy luster, Texture: Cryptocrystalline, Fracture: Conchoidal (shell-like curves), Luster: Waxy to dull, Cleavage: None.

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the seafloor or by chemical precipitation from silica-rich groundwater within limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to sharp edges; used today as road gravel, in concrete, and as high-quality flint for fire-starting gear.

Geological facts

Chert is so hard it can scratch glass. It was one of the first materials traded over long distances by prehistoric humans for tool-making. The gray/black variety is commonly referred to as flint.

Field identification & locations

Look for the characteristic conchoidal fracture (smooth, curved breaks) and a waxy surface texture. It is extremely common in creek beds, glacial till, and embedded in limestone outcrops worldwide.