Chert (Nodule)

Physical Properties

Hardness: 7 (Mohs scale); Color: Varying shades of gray, brown, and tan with a yellowish weathered cortex; Luster: Waxy to dull; Structure: Microcrystalline or cryptocrystalline; 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 represents Cretaceous or older geological formations.

Uses & Applications

Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, construction aggregate, and occasionally in jewelry as tumble-polished stones.

Geological Facts

Chert is so hard it can strike sparks against steel, a property that led to its use in flintlock firearms. It is chemically identical to quartz but has a different microscopic crystal structure.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or weathered out of carbonate rock outcrops.

Identified on: 4/22/2026

Mode: Standard