Chert (Iron-stained)

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Tan, yellow-brown due to iron oxide staining. Luster: Dull to waxy. Structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.6-2.7.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or inorganic precipitation from silica-rich fluids within sedimentary rock layers, often limestone or dolostone.

Uses & Applications

Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracturing. Modernly used in road construction, landscaping, and as a polishing abrasive.

Geological Facts

Chert was one of the first materials used by early humans for fire-starting and tool-making because it produces sharp edges and sparks when struck against steel or iron pyrites.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel blade) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or within limestone outcrops.

Identified on: 4/18/2026

Mode: Standard