Chert (Flint)

Physical Properties

Hardness: 7 (Mohs scale); Color: Black, grey, dark blue; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed from the accumulation of microscopic silica-rich organisms (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.

Uses & Applications

Historically used for stone tools and starting fires; modernly used in road construction, ceramics, and as decorative gravel.

Geological Facts

Dark-colored chert is specifically called Flint. It was the primary material for the Stone Age tool revolution due to its predictable conchoidal fracturing which creates razor-sharp edges.

Field Identification & Locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and its characteristic shell-like (conchoidal) fracture patterns when broken.

Identified on: 4/18/2026

Mode: Standard