sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: dark gray, black, or brown; Luster: waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: none; Fracture: Conchoidal; Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: dark gray, black, or brown
Luster: waxy to dull

Identified July 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or by chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and to start fires (flint and steel). Currently used in road construction, as jewelry (when patterned), and as a grinding medium in ball mills.

Geological facts

Black chert is specifically referred to as 'Flint'. Because it breaks with a conchoidal (shell-like) fracture, it creates edges that are sharper than a surgeon's scalpel, which is why it was the primary material for prehistoric tools.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and the way it flakes into sharp, curved edges when struck. Commonly found in chalk or limestone outcrops and along beaches or riverbeds.