Chert (often called Flint)

Physical Properties

Hardness: 7 (Mohs scale); Color: Gray, tan, or brown with possible banding; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (exhibits prominent conchoidal fracture); Specific gravity: 2.6 – 2.7.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or the chemical precipitation from silica-rich groundwater within limestone or chalk beds. Most specimens originate in Paleozoic to Mesozoic sedimentary basins.

Uses & Applications

Historically used for stone tools and weapons due to its sharp edges; industrially used as an abrasive or in glassmaking; occasionally used in landscape construction or as decorative jewelry (Jasper varieties).

Geological Facts

Chert was one of the most important minerals for early humans, used to create arrowheads and scrapers. When struck against steel, it produces sparks, making it vital for early fire-starting and flintlock firearms.

Field Identification & Locations

Identify in the field by its very smooth, waxy surface and curved, shell-like (conchoidal) fractures when broken. It cannot be scratched by a steel knife. Commonly found in nodules within limestone outcrops or as river-worn pebbles.

Identified on: 4/20/2026

Mode: Standard