sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs), Color: Grey, brown, or tan, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified June 12, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs), Color: Grey, brown, or tan, Luster: Waxy to dull, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed by the precipitation of silica from aqueous fluids or the accumulation of siliceous remains such as radiolaria or diatoms in marine or freshwater environments. It often occurs as nodules within limestone or chalk.

Uses & applications

Historically used for making stone tools (arrowheads, knives) due to its sharp edges; used in industrial grinding media, and as a starting material for fire-making (flint and steel).

Geological facts

Chert is essentially the same chemical composition as quartz but has a much finer grain size. It was one of the first materials used by early humans for tool development.

Field identification & locations

Identified in the field by its very hard surface (scratches glass), waxy feel, and smooth, curved (conchoidal) fracture surfaces like broken glass. Commonly found in river beds or chalk outcrops.