Limonite Concretion

Physical Properties

Hardness: 4-5.5; Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Structure: Amorphous/Microcrystalline; Opaque with a centered cavity.

Formation & Geological History

Formed by the precipitation of iron oxide minerals from groundwater within sedimentary rock layers, often around a host nucleus or through the weathering of iron-rich minerals. Common in Mesozoic and Cenozoic age sediments.

Uses & Applications

Primarily of interest to collectors and educators as a curiosity; historically used as a low-grade iron ore (limonite).

Geological Facts

These are sometimes called 'rattle stones' (Klapperstein) if the internal core is loose and makes noise when shaken. They are common in iron-rich sandstone formations.

Field Identification & Locations

Identify by the rusty yellow-brown color, heavy iron weight relative to size, and hollow or multi-layered 'onion' structure. Frequently found in stream beds or eroded hillsides.

Identified on: 5/19/2026

Mode: Standard