Chert (Nodule)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, brown, tan; Luster: Waxy to dull; Crystal structure: Hexagonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed by the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or through chemical precipitation in marine or groundwater environments. Can occur as nodules in limestone or chalk.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative garden stone, in construction aggregates, or as a fire-starter flint.

Geological Facts

Chert is extremely durable and chemically resistant. Flint is a specific dark variety of chert. It has been used by humans for millions of years to create tools and start fires.

Field Identification & Locations

Identify in the field by its very hard surface (scratches glass), waxy feel, and smooth, shell-like (conchoidal) fractures when broken. Commonly found in limestone outcrops or riverbeds.

Identified on: 4/24/2026

Mode: Standard