sedimentary

Chert (specifically Gray/Blue-Gray Chert)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, bluish-gray, to brownish; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline (Trigonal); Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, bluish-gray, to brownish
Luster: Waxy to dull

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the accumulation of siliceous remains (like diatoms or radiolarians) or via chemical precipitation of silica-rich fluids within limestone or chalk beds. Often dated from the Precambrian to Cenozoic eras.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include road surfacing, decorative landscaping, and as an industrial abrasive or source of silica.

Geological facts

Chert is so hard it can create sparks when struck against steel, which made it vital for prehistoric survival and later for flintlock firearms. It is chemically identical to quartz but contains microscopic impurities that give it an opaque appearance.

Field identification & locations

Identify in the field by its characteristic 'clamshell' (conchoidal) fractures and its ability to scratch glass. It feels smoother or 'waxier' than most other sedimentary rocks. Found globally in sedimentary basins, particularly in the midwestern United States and parts of Europe.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Sandstone

Arenite (primarily SiO2)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

sedimentary

Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary