Chert (Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, white, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & Geological History

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

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) and starting fires; currently used as a gemstone (jaspers), in road construction, and for decorative landscaping.

Geological Facts

Flint is a variety of chert that was essential to prehistoric humans because it breaks into very sharp edges through conchoidal fracturing. It can generate sparks when struck against steel.

Field Identification & Locations

Identify by its extreme hardness (scratches glass), waxy texture, and smooth, curved shell-like (conchoidal) fractures at broken edges. Commonly found in nodular forms within limestone layers.

Identified on: 5/1/2026

Mode: Standard