Chert (Flint)

Physical Properties

Hardness: 7 (Mohs scale), Color: Dark brown to black, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Formation & Geological History

Formed as nodules or layers in limestone or chalk through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) that dissolve and reprecipitate as microcrystalline quartz.

Uses & Applications

Historically used for stone tools and starting fires; currently used as construction aggregate, road surfacing, and occasionally in lapidary work.

Geological Facts

Flint was one of the most important minerals for early humans, used to create spearheads and arrowheads due to its predictable conchoidal fracturing and sharp edges.

Field Identification & Locations

Identify by its extreme hardness, waxy texture, and shell-like (conchoidal) fractures. Often found in chalk cliffs or as stream-rolled nodules. Common in the UK, USA, and Europe.

Identified on: 5/1/2026

Mode: Standard