sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 6.5-7 Mohs; Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.65

Hardness: 6
Color: Gray, tan, or brown
Luster: Waxy to dull

Identified July 1, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Gray, tan, or brown; Luster: Waxy to dull; Structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.65

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of limestone/chalk by silica-rich fluids. Often found as nodules in carbonate rocks.

Uses & applications

Historically used for stone tools and weapons (arrowheads); currently used in road construction, as grinding media, and occasionally as an ornamental stone.

Geological facts

Chert has been used by humans for over 2 million years. Its ability to break with very sharp edges (conchoidal fracture) made it the premier material for prehistoric toolmaking.

Field identification & locations

Identify by its waxy appearance, extreme hardness (it will scratch glass), and the way it breaks into curved, shell-like flakes. Commonly found in limestone beds and river gravels.