sedimentary

Chert (Flint)

Chert (Cryptocrystalline Silicon Dioxide, SiO2)

Hardness: 6.5-7 Mohs; Color: Gray, black, or brown with a white weathered cortex; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 6
Color: Gray, black, or brown with a white weathered cortex
Luster: Waxy to dull

Identified June 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily through the chemical precipitation of silica-rich fluids or the accumulation of siliceous skeletal remains (like sponges or diatoms) within limestone or chalk beds, often during the Cretaceous period.

Uses & applications

Historically used for stone tools and weapons due to its sharp edges; modernly used as road aggregate, in glass making, and occasionally as decorative landscaping stone.

Geological facts

Flint was one of the first materials used by early humans to create fire and tools. It produces sparks when struck against steel. The white outer layer is called a 'cortex' and is caused by weathering and mineral leaching.

Field identification & locations

Identify in the field by its very smooth, glass-like interior revealed by chips, its conchoidal (shell-like) fracture patterns, and its ability to scratch glass. Commonly found in chalk or limestone outcrops and on beaches in the UK and Northern Europe.

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