Chert

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, tan, white, brown with iron staining; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed primarily in marine environments through the accumulation of siliceous remains of organisms like diatoms or radiolarians, or via chemical precipitation from silica-rich fluids. Can be found in various geological ages.

Uses & Applications

Historically used for making sharp tools like arrowheads and scrapers. Currently used in road construction, as an abrasive, and occasionally for lapidary work.

Geological Facts

Chert is so hard it can scratch steel and produce sparks when struck against iron. Flint is a well-known dark variety of chert.

Field Identification & Locations

Identify in the field by its extreme hardness, conchoidal (shell-like) fracture patterns, and waxy texture. Frequently found as nodules within limestone or dolomite beds.

Identified on: 5/1/2026

Mode: Standard