sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 7 (Mohs); Color: Grey, brown, tan, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 7 (Mohs)
Color: Grey, brown, tan, or black
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs); Color: Grey, brown, tan, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or via chemical precipitation from silica-rich fluids in sedimentary basins. Often found as nodules in limestone or chalk.

Uses & applications

Historical use for stone tools and weapons (arrowheads); industrial use in road surfacing and as a gemstone component; historically used to start fires.

Geological facts

Chert has been used by humans for over two million years to create sharp-edged tools. It is so tough that it was the primary material for the 'Stone Age'.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture, wax-like luster, and its ability to scratch glass. Commonly found in riverbeds or eroding out of limestone cliffs.