sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 6.5-7 Mohs; Color: Gray, black, or brown; Luster: Waxy or dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.

Hardness: 6
Color: Gray, black, or brown
Luster: Waxy or dull

Identified May 4, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Gray, black, or brown; Luster: Waxy or dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8.

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like sponges or radiolarians) or chemical precipitation in sedimentary basins; primarily found in limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, knives); currently used in road construction, and occasionally in jewelry as cabochons.

Geological facts

Flint was a vital resource for prehistoric humans because its conchoidal fracture produces extremely sharp edges. It can also produce sparks when struck against steel.

Field identification & locations

Look for a microcrystalline texture, conchoidal (shell-like) fractures, and a hardness that can scratch glass. Common in chalk formations or coastal gravels.