Ferruginous Chert (Jasper)

Physical Properties

Hardness: 6.5-7 on Mohs scale; Color: Reddish-brown, earthy orange, and dull green; Luster: Waxy to dull/earthy; Crystal structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.9

Formation & Geological History

Formed through the precipitation of silica from groundwater or in marine environments, often associated with banded iron formations or volcanic activity. Iron oxides create the reddish tones, while other minerals/clays provide the green hues.

Uses & Applications

Commonly used for lapidary work (cabochons, beads), decorative garden stones, and historically used by indigenous people for stone tools due to its sharp fracture.

Geological Facts

Chert is so durable that it often survives deep within topsoil long after the surrounding rock has weathered away. Some specimens are billions of years old, dating back to the Precambrian era.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel nail) and its smooth, shell-like fracture pattern. Often found in stream beds, gravel pits, and eroding cliffsides.

Identified on: 4/19/2026

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