Chert (Flint)

Physical Properties

Hardness: 7 (Mohs scale); Color: Gray, black, white, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or sponges) on the ocean floor or through chemical precipitation in groundwater. It is often found as nodules in limestone or chalk beds.

Uses & Applications

Historically used for making stone tools and starting fires (flint); currently used in road construction, as a flux in glassmaking, and occasionally for decorative stone or lapidary work.

Geological Facts

Flint was one of the most important minerals for early humans, used to create razor-sharp arrowheads and scrapers. Because it breaks with a conchoidal fracture, it creates sharp, glass-like edges.

Field Identification & Locations

Identify in the field by its hardness (it scratches glass), its waxy feel, and its smooth, curved (conchoidal) breaks. Often found in chalk cliffs or as nodules in limestone regions.

Identified on: 4/25/2026

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