sedimentary

Ironstone Nodule / Limonite Concretion

Iron-rich sedimentary concretion (FeO(OH)·nH2O / Fe2O3)

Hardness: 4.0-5.5 on Mohs scale; Color: Reddish-brown, yellowish-brown, rust orange; Luster: Earthy, dull; Structure: Amorphous to microcrystalline concretionary crust; Streak: Yellowish-brown to reddish-brown.

Hardness: 4
Color: Reddish-brown, yellowish-brown, rust orange
Luster: Earthy, dull

Identified July 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through precipitation of iron minerals (limonite, goethite, hematite) around a central nucleus within soft sedimentary layers like mudstone or sandstone. The outer iron-rich crust hardens while the core may remain softer or hollow.

Uses & applications

Historically harvested as a minor iron ore; used in pigment production (ochre/sienna) and collected by rockhounding enthusiasts as geological curiosities.

Geological facts

These broken iron-stained nodule fragments reveal an earthy interior coated in rust-like iron oxides, often found along gravelly riverbeds and coastal beaches, including the Caspian coast area (e.g., Dagestan region).

Field identification & locations

Look for rounded, heavy nodules or broken egg-shell-like crusts with distinct rusty yellow-brown oxidation and soft or sandy interiors amidst water-worn river gravel.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sedimentary Sandstone

Arenite (Sedimentary Rock)

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Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

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Shale

Shale (clastic sedimentary rock)

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Shale

Argillaceous sedimentary rock

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Mudstone

Argillite / Siliciclastic sedimentary rock

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