Chert

Physical Properties

Hardness: 7 (Mohs scale), Color: Brown, tan, and yellowish-orange (iron-stained), Luster: Dull to waxy, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.5-2.8

Formation & Geological History

Formed by the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. It can range from Precambrian to modern in age.

Uses & Applications

Historically used for stone tools (flintknapping) due to its sharp edges; currently used for road surfacing, construction aggregate, and occasionally as an ornamental garden stone.

Geological Facts

Chert has been used by humans for millions of years to create arrowheads, scrapers, and knives. It is so hard that it can spark when struck against steel.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture. It is commonly found as nodules in limestone regions or as stream-worn pebbles.

Identified on: 4/27/2026

Mode: Standard