Chert (Nodule)

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.

Identified on: 4/24/2026

Mode: Standard