sedimentary

Chert (Nodule)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Brown, gray, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Brown, gray, tan
Luster: Waxy to dull

Identified April 24, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Brown, gray, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Often Cretaceous or older.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp fracture edges. Today used in construction aggregate and as a collector's specimen.

Geological facts

Chert was one of the most important minerals for early humans because its conchoidal fracture allows it to be chipped into extremely sharp edges. Flint is a variety of dark-colored chert.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and waxy, smooth texture compared to the surrounding rock. Often found as lumpy nodules in limestone outcrops or riverbeds.