Chert (Basanite / Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Dull to waxy; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. This specific specimen appears to be a river-worn or beach-worn pebble.

Uses & Applications

Historically used for making stone tools (arrowheads/scrapers) due to conchoidal fracture. Industrially used as an abrasive or in construction aggregate. Dark varieties (Basanite) are used as 'touchstones' to test precious metals.

Geological Facts

Black chert is often called 'Lydian stone' or 'Basanite' and was used by ancient goldsmiths to determine the purity of gold by comparing the color of the streak left on the stone.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Common on beaches and in stream beds.

Identified on: 4/22/2026

Mode: Standard