sedimentary

Chert (Flint Nodules)

Chert (Microcrystalline Quartz, SiO2)

Hardness: 7 (Mohs); Color: Grey, black, or brown with white exterior (cortex); Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs)
Color: Grey, black, or brown with white exterior (cortex)
Luster: Waxy to dull

Identified April 3, 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 in marine environments. These nodules often grow within limestone or chalk beds as silica replaces carbonate material.

Uses & applications

Used historically for stone tools and starting fires (striking steel). Modern uses include road surfacing, decorative landscaping, and occasionally as semi-precious gemstones when patterned.

Geological facts

Flint was the primary material for the Stone Age 'industry' due to its ability to be knapped into razor-sharp edges. It is famous for creating sparks when struck against iron or pyrite.

Field identification & locations

Identify by its 'skin' or cortex, waxy interior when broken, and characteristic shell-like (conchoidal) fractures. Often found in chalk cliffs, riverbeds, and coastal beaches where sedimentary rocks have eroded.

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