sedimentary

Chert (specifically Flint or Dark Chert)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Grey, black, or dark brown with white weathering rinds; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Grey, black, or dark brown with white weathering rinds
Luster: Dull to waxy

Identified April 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from seawater or the replacement of limestone/chalk by silica-rich fluids. Often contains fossilized remains of microorganisms. Common in sedimentary basins worldwide.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) and starting fires (flint and steel). Currently used as road aggregate, in glass manufacturing, and as decorative gravel.

Geological facts

Chert is so hard it can scratch steel. Because it breaks with sharp, curved edges (conchoidal fracture), it was the primary material for prehistoric weaponry and tools for millions of years.

Field identification & locations

Identify in the field by its smooth, shell-like fracture surfaces and extreme hardness. It is frequently found as nodules within limestone or as rounded pebbles on beaches and in riverbeds.

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Sandstone or Siltstone

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Sandstone

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Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

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Sandstone (with man-made markings)

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