Chert (with Iron Staining)

Physical Properties

Hardness: 7 on Mohs scale; Color: typically gray, green, or brown (often with red/orange iron oxidation); Luster: waxy to dull; Structure: microcrystalline; Cleavage: none (conchoidal fracture); Specific Gravity: 2.65.

Formation & Geological History

Formed through the precipitation of silica from seawater or the replacement of limestone by silica-rich fluids. Often contains the remains of microscopic organisms like diatoms or radiolarians. Found in sedimentary deposits of various geological ages.

Uses & Applications

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

Geological Facts

Chert was one of the most important minerals for early humans because its sharp edges were ideal for cutting and hunting. When struck against steel, it can produce sparks, used for thousands of years to start fires.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, glass-like (conchoidal) fracture surfaces. Common in river beds and limestone outcrops.

Identified on: 4/13/2026

Mode: Standard