sedimentary

Chert

Chert (Microcrystalline Silica, SiO2)

Hardness: 6.5-7 (Mohs scale), Color: Brownish-gray, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.6-2.65

Hardness: 6

Identified July 17, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 (Mohs scale), Color: Brownish-gray, Luster: Dull to waxy, Crystal Structure: Cryptocrystalline, Fracture: Conchoidal, Specific Gravity: 2.6-2.65

Formation & geological history

Formed in marine environments through the precipitation of silica from seawater or from the accumulation of siliceous skeletal remains of microorganisms like diatoms and radiolaria. Found in sedimentary strata worldwide.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Currently used as road aggregate, in concrete, and occasionally as a low-cost lapidary material.

Geological facts

Chert is essentially the same material as flint, but the term 'flint' is often reserved for specimens found in chalk or marly limestone. It is one of the most durable sedimentary rocks.

Field identification & locations

Identified by its extreme hardness (cannot be scratched by a steel knife), smooth waxy feel, and conchoidal (shell-like) fracture edges. Commonly found in riverbeds and limestone outcrops.