Chert (with Limonite staining)

Physical Properties

Hardness: 6.5–7 on Mohs scale; Color: White/Grey with yellow-brown staining; Luster: Dull to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation in marine environments. The yellow staining is likely Limonite, a hydrated iron oxide formed via weathering.

Uses & Applications

Historically used for stone tools and arrowheads due to its sharp fracture edges. Today it is used in road construction, as an abrasive, and sometimes in landscaping.

Geological Facts

Chert is so hard it can scratch glass and steel. Certain varieties like flint have been used for thousands of years to create fire by striking them against steel.

Field Identification & Locations

Identify by its extreme hardness and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone or chalk beds as nodules or layers throughout the world.

Identified on: 4/18/2026

Mode: Standard