Banded Chert

Physical Properties

Hardness: 6.5-7 (Mohs scale); Color: Variegated tan, brown, and grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or groundwater environments. Often associated with ancient limestone or dolostone formations.

Uses & Applications

Used historically for stone tools (lithics) due to its sharp edges; used today as decorative gravel, in lapidary work for cabochons, and as a raw material for silica industrial products.

Geological Facts

Chert was one of the first materials used by early humans to create fire-starting tools and arrowheads. Banding occurs due to slight changes in mineral impurities or depositional rates during formation.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel nail) and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds or within sedimentary layers.

Identified on: 5/3/2026

Mode: Standard