sedimentary

Chert (and Flint variant)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, brown, beige, or white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic hexagonal crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, brown, beige, or white
Luster: Waxy to dull

Identified April 5, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or sponges) on the seafloor or by chemical precipitation of silica in groundwater. Often found as nodules within limestone or chalk formations.

Uses & applications

Historically used for stone tools and starting fires (striking against steel). Modern uses include jewelry (as jasper/agate), building material in some regions, and industrial abrasives.

Geological facts

Chert is so hard it can scratch glass and steel. Because it breaks with a conchoidal (shell-like) fracture, it creates edges as sharp as a razor, making it the primary material for prehistoric arrowheads and spear tips.

Field identification & locations

Identify in the field by its smooth, waxy surface and the way it chips into sharp, curved flakes. It is commonly found in sedimentary rock outcrops, riverbeds, and along shorelines. Collectors look for unique banding or 'eye' patterns.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary