sedimentary

Chert (Flint)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 (Mohs), Color: Grey, tan, brown, white; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs), Color: Grey, tan, brown, white
Color: Grey, tan, brown, white
Luster: Dull to waxy

Identified July 16, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs), Color: Grey, tan, brown, white; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed as nodules or layers in carbonate rocks through the precipitation of silica-rich fluids (often from microfossils like diatoms or radiolaria) during diagenesis.

Uses & applications

Used historically for stone tools and fire-starting; currently used in road construction, as an abrasive, and sometimes for decorative landscape gravel.

Geological facts

Chert has a conchoidal fracture, allowing it to break with sharp edges, which made it the primary material for arrowheads and spear points in prehistoric times.

Field identification & locations

Look for a microcrystalline texture that doesn't show visible grains, a waxy luster on fresh surfaces, and edges that are sharp and glass-like when broken. Common in limestone deposits.