sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs; Color: Gray, brown, and mottled tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs
Color: Gray, brown, and mottled tan
Luster: Waxy to dull

Identified July 21, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Gray, brown, and mottled tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed in marine environments through the precipitation of silica from the skeletal remains of diatoms or sponges, or by chemical replacement during diagenesis in limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads); currently used as road aggregate, architectural stone, and in the production of fire-striking kits.

Geological facts

Because it fractures conchoidally to create extremely sharp edges, chert was one of the most vital materials for early human survival for weaponry and tool-making.

Field identification & locations

Identify in the field by its hardness (will scratch glass), smooth shell-like fracture surfaces, and fine-grained texture. Commonly found as nodules within limestone or as river pebbles.