sedimentary

Chert (Gravel Fragment)

Silicon dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray to brownish-gray; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray to brownish-gray
Luster: Dull to waxy

Identified May 26, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray to brownish-gray; Luster: Dull to waxy; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & geological history

Formed by the precipitation of silica from water or the accumulation of silica-rich organic remains (like diatoms or radiolarians) on the ocean floor. Most are hundreds of millions of years old.

Uses & applications

Commonly used as construction aggregate for driveways and roadbeds. Historically used for stone tools and starting fires (flint).

Geological facts

Chert is chemically very similar to quartz but has a much finer grain structure. Flint is a specific dark variety of chert often found in chalk deposits.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel) and curved, glass-like fracture surfaces. Frequently found in riverbeds and as crushed gravel.