Chert (Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, or grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

Formed through the accumulation of siliceous remains of marine organisms (diatoms, radiolarians) or by chemical precipitation from silica-rich fluids within limestone or chalk beds.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) and to start fires. Modern uses include road surfacing, jewelry (when colorful/polished), and decorative landscape gravel.

Geological Facts

Because it breaks with a conchoidal fracture like glass, it creates edges that are sharp on a molecular level. It has been used by humans for over 2 million years and was the primary material for the Stone Age.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. Often found in riverbeds or weathered out of limestone cliffs.

Identified on: 4/16/2026

Mode: Standard