Chert (Jasper/Flint variety)

Physical Properties

Hardness: 7 (Mohs scale); Color: Dull gray-green with hints of red; Luster: Waxy to dull; Crystal 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 radiolarians or diatoms) or by chemical precipitation in marine environments. Often found as nodules in limestone or chalk.

Uses & Applications

Historically used for making stone tools (arrowheads/scrapers) due to sharp edges; currently used for road surfacing, lapidary work, and as a polishing media.

Geological Facts

Chert has been used by humans for millions of years. Because of its hardness and the way it breaks into razor-sharp edges, it was the primary material for the Stone Age technology. It can produce sparks when struck against steel.

Field Identification & Locations

Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, curved (conchoidal) fracture surfaces. Commonly found in riverbeds and coastal areas as rounded pebbles.

Identified on: 4/22/2026

Mode: Standard