sedimentary

Flint

Chert (SiO2)

Hardness: 7 Mohs; Color: Gray, black, or dark brown interior with a white/grey chalky cortex; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)

Hardness: 7 Mohs
Luster: Waxy to vitreous

Identified June 14, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Gray, black, or dark brown interior with a white/grey chalky cortex; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)

Formation & geological history

Formed as nodules in sedimentary rocks like chalk or marine limestones. It results from the replacement of calcium carbonate by silica during diagenesis, often around organic nuclei like sponges.

Uses & applications

Historically used for stone tools and weapons (knapping) and starting fires. Modern uses include road construction, ceramics, and some specialized abrasive tools.

Geological facts

Flint was the primary material for the Stone Age technology. When struck against steel, it produces a spark because the hard flint shaves off tiny particles of iron that spontaneously ignite in the air.

Field identification & locations

Identify by its smooth, glass-like interior revealed by chips, its extreme hardness, and its characteristic curved (conchoidal) breaks. Often found in areas with chalk deposits or as rounded pebbles on beaches.