sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 Mohs; Color: Brown, tan, or grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs
Color: Brown, tan, or grey
Luster: Waxy to dull

Identified March 17, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Brown, tan, or grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed through the precipitation of silica-rich fluids within sedimentary rock layers like limestone or chalk. Often organic in origin, resulting from the accumulation of microscopic organisms like diatoms or radiolarians, typically during the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for making stone tools (arrowheads/scrapers) due to its sharp fracture edges. Modernly used as an abrasive, in construction aggregate, or as a decorative landscaping stone.

Geological facts

Chert and flint are closely related; flint is essentially high-quality dark chert. It was one of the first materials traded by prehistoric humans over long distances because of its vital role in survival tools and fire-starting.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its curved, shell-like (conchoidal) breaks. It is ubiquitous in limestone outcrops and riverbeds. Collectors look for unique colors or fossil inclusions.