sedimentary

Ironstone Concretion / Limonite Nodule

Limonite / Goethite-cemented Concretion (FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4 to 5.5 on Mohs scale; Color: yellowish-brown, dark brown, rusty tan; Luster: dull, earthy, occasionally submetallic; Streak: yellowish-brown; Fracture: uneven to subconchoidal.

Hardness: 4 to 5
Color: yellowish-brown, dark brown, rusty tan
Luster: dull, earthy, occasionally submetallic

Identified October 5, 2026More sedimentary →

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Physical properties

Hardness: 4 to 5.5 on Mohs scale; Color: yellowish-brown, dark brown, rusty tan; Luster: dull, earthy, occasionally submetallic; Streak: yellowish-brown; Fracture: uneven to subconchoidal.

Formation & geological history

Formed by the precipitation of iron oxides and hydroxides (such as limonite, goethite, or hematite) within sedimentary pores or around organic matter in groundwater or aquatic sedimentary environments.

Uses & applications

Historically collected as minor iron ore; used as natural earth pigments (yellow/brown ochre); collected by rockhounds and fossil enthusiasts (often cracked open to check for fossils).

Geological facts

Iron concretions can take bizarre, rounded, or hollow shapes; when hollow with loose sediment or sand rattling inside, they are popularly called 'rattle stones' or 'Indian paint pots'.

Field identification & locations

Commonly found weathering out of sedimentary strata like shale or sandstone, riverbeds, and gravel bars. Identified by rusty coloration, dense weight relative to standard sandstone, and earthy yellow-brown streak.