Chert (Flint)

Physical Properties

Hardness: 7 Mohs scale; Color: Dark grey to black with brown patina; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formation occurs as nodules or beds in limestone or chalk through the replacement of calcium carbonate by silica-rich fluids. Often formed during the Cretaceous period or earlier.

Uses & Applications

Used historically for stone tools (arrowheads, scrapers) due to sharp edges; currently used as a decorative stone, in construction aggregate, and as a striker for fire-starting.

Geological Facts

Flint is a variety of chert that occurs primarily in chalk and marly limestone. It was one of the most vital materials for early humans to create survival tools.

Field Identification & Locations

Identify by its conchoidal (shell-like) fracture patterns and ability to scratch glass. Commonly found in riverbeds, coastal chalk cliffs, and glacial till.

Identified on: 5/20/2026

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