Chert (Iron-stained)

Physical Properties

Hardness: 7 (Mohs); Color: Brown, tan, and white; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Fracture: Conchoidal; Specific gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. This specimen shows iron oxidation (rusting) giving it a brown hue.

Uses & Applications

Historical use for stone tools and flintknapping. Modern use in construction aggregate, road base, and occasionally as an ornamental garden stone.

Geological Facts

Chert is so hard it can scratch glass and steel. Historically, varieties like flint were essential for starting fires and weaponry due to their sharp, predictable fracture patterns.

Field Identification & Locations

Identify by its extreme hardness and smooth, shell-like fracture surfaces if broken. It is often found as nodules or layers within sedimentary rock outcrops or as river-worn cobbles.

Identified on: 4/18/2026

Mode: Standard