sedimentary

Chert (and Quartzite)

Microcrystalline silica (SiO2)

Hardness: 7 (Mohs scale); Color: White, tan, and brownish-grey; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: White, tan, and brownish-grey
Luster: Waxy to vitreous

Identified May 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in sedimentary basins. These specimens appear to be stream-worn or weathered nodules.

Uses & applications

Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracture. Modernly used as road aggregate, architectural stone, and occasionally as semi-precious gemstones when colorful (Jasper).

Geological facts

Chert was one of the first materials used by early humans to create fire and sharp tools. It is extremely chemically stable and resistant to weathering, which is why these pieces remain intact while surrounding softer rocks erode.

Field identification & locations

Identify by its ability to scratch glass and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone beds or as loose gravel in riverbeds. These specific pieces show a mix of massive quartz and chert characteristics.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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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

Arenite

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