Ferruginous Chert

Physical Properties

Hardness: 6.5-7 on Mohs scale. Color: Tan, yellow-orange to reddish-brown due to iron staining. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6-2.7.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or groundwater environments. The iron staining occurs during or after lithification when iron-rich minerals oxidize over millions of years.

Uses & Applications

Historically used for stone tools and fire-starting (flint). Currently used as landscaping rock, construction aggregate, and occasionally tumbled for lapidary jewelry or educational collections.

Geological Facts

Chert is so durable that it often remains as 'river gravel' long after the surrounding limestone or dolomite bedrock has weathered away. The reddish-brown crust on this specimen is likely limonite or goethite.

Field Identification & Locations

Identify by its hardness (will scratch glass), conchoidal (shell-like) fracture, and smooth, waxy surface. Commonly found in gravel beds, riverbanks, and sedimentary outcroppings.

Identified on: 4/17/2026

Mode: Standard