Chert (variously Flint)

Physical Properties

Hardness: 7 (Mohs scale), Color: Dark gray to bluish-gray with tan/brown weathering, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds.

Uses & Applications

Historically used for stone tools and fire-starting; industrial uses include silica sources for glass; construction aggregate; occasionally used in jewelry or as pocket stones.

Geological Facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a steel scalpel. Native Americans and early humans worldwide used it extensively for arrowheads and scrapers.

Field Identification & Locations

Identify in the field by high hardness (scratches glass), waxy appearance, and sharp, curved (conchoidal) edges when broken. Extremely common in stream beds and coastal cliffs.

Identified on: 4/26/2026

Mode: Standard