sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Variegated grey, tan, and yellow-brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Variegated grey, tan, and yellow-brown
Luster: Waxy to dull

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in sedimentary environments. Common in limestone and chalk beds, often dating from various geological eras including the Cretaceous.

Uses & applications

Historically used for stone tools and starting fires (striking steel). Modernly used in road construction, as jewelry (when polished), and as a silica source in glassmaking.

Geological facts

Chert was one of the most vital resources for prehistoric humans due to its ability to fracture into surgical-sharp edges. It is chemically identical to quartz but has a different microscopic structure.

Field identification & locations

Identify in the field by its distinctive 'conchoidal' (shell-like) fracture edges and waxy texture. It cannot be scratched by a steel pocketknife. Found globally, especially in riverbeds or eroding out of limestone cliffs.