Chert (Black Chert / Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Black to dark gray; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); 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 chemical precipitation in marine environments. It occurs as nodules or layered beds within limestone or chalk formations.

Uses & Applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges. Currently used in road surfacing, as grinding media, and occasionally in jewelry as lapidary material.

Geological Facts

Black chert is often called 'flint' when found in chalk or marly limestone. It was the primary material for the 'Stone Age' technology because its conchoidal fracture creates edges sharper than a steel scalpel.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel pocket knife) and its smooth, shell-like (conchoidal) fracture surfaces. It will often produce sparks when struck against steel.

Identified on: 4/19/2026

Mode: Standard