Chert (Nodule)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan to light brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed in sedimentary environments from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Often spans millions of years in age.

Uses & Applications

Historically used for stone tools due to sharp edges when fractured; currently used for construction aggregate, road surfacing, and as decorative landscape gravel.

Geological Facts

Because it breaks with a conchoidal (shell-like) fracture pattern, chert was one of the most important materials for early humans to create arrowheads, scrapers, and knives.

Field Identification & Locations

Identify by its hardness (it will scratch glass), its waxy feel, and the smooth, curved surfaces where it has chipped or broken. Commonly found in riverbeds or eroding out of limestone outcrops.

Identified on: 5/5/2026

Mode: Standard