sedimentary
Limonite Concretion
Goethite-rich Concretion (FeO(OH))
Hardness: 4-5.5; Color: Yellowish-brown to dark brown; Luster: Dull/Earthy; Structure: Amorphous/Microcrystalline; Opaque with a centered cavity.
- Hardness
- 4-5
- Color
- Yellowish-brown to dark brown
- Luster
- Dull/Earthy
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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.
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