sedimentary

Chert (specifically Flint or Gray Chert)

Silicon Dioxide (Microcrystalline Quartz), SiO2

Hardness: 6.5–7 on the Mohs scale. Color: Gray, tan, and brownish-black banding. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.

Hardness: 6
Color: Gray, tan, and brownish-black banding
Luster: Waxy to dull

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica in sedimentary basins, often replacing organic matter or limestone/chalk through diagenesis. Frequently found in marine environments as nodules or bedded layers. Geological age varies widely from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools and weapons due to sharp edges; currently used as construction aggregate, road base, and as high-quality flint for fire-starting or polishing in jewelry (lapidary).

Geological facts

Chert was one of the most important minerals to early humans for making arrowheads and spear points because of its predictable conchoidal fracture, which results in razor-sharp edges.

Field identification & locations

Identify in the field by looking for a smooth, glass-like fracture (conchoidal) and a waxy surface. It will scratch glass easily. Common in riverbeds, limestone outcrops, and coastal cliffs across North America and Europe.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

Arenite (primarily SiO2)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

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Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

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