Ferruginous Chert

Physical Properties

Hardness: 7 (Mohs scale); Color: Dark reddish-brown to maroon; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & Geological History

Formed through the chemical precipitation of silica-rich fluids in sedimentary environments. The dark red color is due to the presence of hematite or other iron oxides during formation.

Uses & Applications

Used historically for stone tools due to its conchoidal fracture; modernly used as decorative landscaping rock or for lapidary work like cabochons.

Geological Facts

Commonly associated with Banded Iron Formations (BIFs) which date back over 2 billion years; it is a primary source for understanding early Earth atmospheric conditions.

Field Identification & Locations

Identify by its hardness (it will scratch glass), its smooth, shell-like fracture surfaces, and its high density compared to some other sedimentary rocks. Commonly found in creek beds or glacial till near banded iron deposits.

Identified on: 4/29/2026

Mode: Standard