Chert (specifically Gray Chert)

Physical Properties

Hardness: 7 (Mohs scale), Color: light to dark gray/tan, Luster: waxy to dull, Crystal structure: microcrystalline/cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65

Formation & Geological History

Formed from the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through chemical precipitation in groundwater. Often occurs as nodules or layers within limestone or shale.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone, in road construction, and as flint for fire-starting.

Geological Facts

Chert has been used by humans for over 2 million years. When it is distinctively colorful, it is often called Jasper; when it is dark and high in organic matter, it is called Flint.

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 limestone outcrops, river beds, and gravel pits.

Identified on: 4/24/2026

Mode: Standard