sedimentary

Weathered iron-rich concretion

Limonite/goethite-bearing ferruginous concretion, commonly forming in sandstone or alluvial deposits

AI-generated identification · may be incorrect

Hardness about 4–5.5 Mohs; rusty brown to dark brown, earthy to dull/submetallic luster, irregular massive structure, and no obvious cleavage. The rounded, pitted surface and dark metallic patches suggest iron oxides coating or cementing a harder host rock.

Identified September 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when groundwater precipitates iron oxides and hydroxides around a nucleus, often during weathering or sediment compaction; individual concretions may range from recent to millions of years old. The rounded shape may also reflect stream abrasion.

Uses & applications

Primarily a teaching or collecting specimen; iron-rich concretions have limited industrial significance, while related limonite/goethite ores have historically been mined for iron and pigments.

Geological facts

Iron oxides produce the characteristic ochre-brown color and can develop botryoidal or knobby textures. A streak test may yield yellow-brown to reddish-brown powder.

Field identification & locations

Common in weathered sediments, river gravels, soils, and sandstone exposures worldwide. Identification is supported by high density, rusty staining, an ochre streak, and attraction to a strong magnet if magnetite is present.