sedimentary

Flint

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Black, grey, or dark brown with a white chalky cortex; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Black, grey, or dark brown with a white chalky cortex
Luster: Dull to waxy

Identified July 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms as nodules or masses in sedimentary rocks like chalk and limestone; created through diagenesis where silica-rich fluids replace carbonate material over millions of years

Uses & applications

Used historically for stone tools and starting fires; currently used in jewelry (as striped flint), high-quality road construction, and fire-striking kits

Geological facts

Flint was the primary material for the Stone Age 'industry' because of its ability to break into razor-sharp edges. It produces sparks when struck against steel due to the friction oxidizing iron particles.

Field identification & locations

Identify by its glassy, shell-like (conchoidal) fracture surfaces and its white exterior 'skin' or cortex. Commonly found in the chalk cliffs of Dover (UK), France, and across the Midwestern USA.