Chert

Physical Properties

Hardness of 6.5-7 on the Mohs scale. Color varies widely but often gray, brown, tan, or black. Luster is dull to waxy. Microcrystalline structure. Exhibits conchoidal fracture. Specific gravity is approximately 2.6.

Formation & Geological History

Forms as a chemical precipitate or from the accumulation of silica-rich marine organisms (like diatoms or radiolarians) in deep ocean environments, or as nodules in limestone or chalk networks. Can be of various geological ages.

Uses & Applications

Historically used for making stone tools (like arrowheads and scrapers) due to its conchoidal fracture. Sometimes used as a road surfacing material or in flintlock firearms when found as flint.

Geological Facts

Flint is a dark variety of chert often found as nodules in chalk. Chert has been crucial in human history for tool-making.

Field Identification & Locations

Identified by its hardness (scratches glass), waxy luster, and smooth, curved (conchoidal) fracture surfaces. Commonly found worldwide in sedimentary rock sequences.

Identified on: 4/26/2026

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