Chert (Basanite / Flint variety)

Physical Properties

Hardness: 7 (Mohs scale); Color: Black to dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary host rocks. Common in limestone and chalk beds.

Uses & Applications

Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, friction fire-starting, and as decorative river stones.

Geological Facts

Dark black chert is often referred to as 'Basanite' or 'Touchstone' and was historically used by jewelers to test the purity of gold by observing the color of the streak left on the stone.

Field Identification & Locations

Identify by its 'glassy' smooth texture and curved, shell-like (conchoidal) fractures when broken. Commonly found in riverbeds and coastal areas as rounded pebbles.

Identified on: 4/20/2026

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