Chert (Basanite / Flint)

Physical Properties

Hardness: 6.5–7; Color: Dark grey to black; Luster: Dull to waxy; Structure: Cryptocrystalline/Amorphous; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & Geological History

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

Uses & Applications

Historically used for stone tools/weapons due to sharp edges; currently used for road surfacing, fire-starting (striking steel), and as decorative landscaping stones.

Geological Facts

Black chert is often referred to as 'Basanite' or 'Touchstone' and was historically used by goldsmiths to test the purity of gold by observing the color of the streak left behind.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or coastal erosion zones.

Identified on: 4/23/2026

Mode: Standard