sedimentary

Chert (and Flint)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Ranges from gray, brown, and tan to reddish-brown; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5–2.8.

Hardness: 7 (Mohs scale)
Color: Ranges from gray, brown, and tan to reddish-brown
Luster: Dull to waxy

Identified May 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from silica-rich fluids within other sedimentary rocks like limestone. They can form as nodules or bedded layers.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to sharp conchoidal edges; used today as a heavy-duty road base, in gemstone tumbling, and as flint for survival fire-starting kits.

Geological facts

Chert was one of the most important minerals for early humans, used for several millennia to create sharp-edged tools. When struck against steel, it produces a spark, making it essential for early firearms (flintlocks) and fire-starting.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its distinct shell-like (conchoidal) fracture pattern. Found globally in limestone formations and as river pebbles.