sedimentary

Chert / Flint with Cortex

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: White outer chalky cortex with grey or tan nodules; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: White outer chalky cortex with grey or tan nodules
Luster: Dull to waxy

Identified May 7, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: White outer chalky cortex with grey or tan nodules; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed as nodules in limestone or chalk deposits through the replacement of calcium carbonate with silica-rich fluids. Generally dates from the Cretaceous period (66-145 million years ago).

Uses & applications

Historically used for stone tools and starting fires; currently used in construction as aggregate and occasionally as semi-precious gemstones for jewelry (jasper/agate varieties).

Geological facts

The white outer layer, called the cortex, is a porous zone resulting from weathering or incomplete silica replacement of the surrounding limestone. Flint has been used by humans for over 2 million years.

Field identification & locations

Look for a smooth, glass-like interior if broken, and a sharp, shell-like (conchoidal) fracture pattern. It will easily scratch glass. Found frequently in the White Cliffs of Dover and chalk beds throughout Europe.