Chert (River Pebble)

Physical Properties

Hardness: 7 (Mohs scale); Color: Variegated grey, tan, and cream; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline/cryptocrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from seawater. This specimen has been smoothed and rounded by transport in a river or high-energy water environment.

Uses & Applications

Historically used for stone tools (flintknapping) due to its sharp edges. Currently used as decorative gravel, in construction as aggregate, and sometimes polished for jewelry or tumbled stones.

Geological Facts

Chert is so hard it can spark when struck against steel. It was one of the most vital materials for prehistoric humans for making arrowheads and spear points.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, waxy surface. It often breaks with a shell-like (conchoidal) curve. Commonly found in riverbeds and limestone outcrops.

Identified on: 4/27/2026

Mode: Standard