sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, blue-gray, brown, tan, and white; Luster: Waxy to dull; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Gray, blue-gray, brown, tan, and white
Luster: Waxy to dull

Identified July 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids within sedimentary rock layers, often limestone or chalk.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used for road surfacing, as an abrasive, and sometimes in jewelry like cabochons.

Geological facts

Chert was one of the most important minerals for early humans because its conchoidal fracture creates extremely sharp edges for spear points and knives. It can also create sparks when struck against steel.

Field identification & locations

Look for a smooth, waxy texture and shell-like (conchoidal) fractures. It is commonly found in riverbeds, limestone outcrops, and as nodules in chalk. It will scratch glass easily.