Chert with Iron Staining

Physical Properties

Hardness: 7 (Mohs scale); Color: Dark gray to black matrix with reddish-brown (limonite/hematite) iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed as a chemical precipitate from silica-rich fluids or the accumulation of siliceous organic remains (radiolarians/diatoms) in marine environments. The iron staining occurs via oxidation in groundwater.

Uses & Applications

Historically used for stone tools (arrowheads/knives) due to sharp edges; currently used for road ballast, abrasives, or as lapidary rough for polishing.

Geological Facts

Chert was one of the first materials utilized by early humans for fire-starting and tool-making. It is so tough it will easily scratch glass and even steel.

Field Identification & Locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its extreme hardness. Common in limestone formations and as river pebbles.

Identified on: 5/18/2026

Mode: Standard