sedimentary

Ironstone Concretion

Siderite / Hematite-bearing Sedimentary Rock

Hardness 5.0 to 6.5 on the Mohs scale, earthy to sub-metallic luster, colors ranging from tan and yellowish-brown to dark reddish-brown, massive or concentric nodular structure, variable specific gravity (~2.8 - 3.8).

Identified July 27, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during diagenesis in sedimentary environments when mineral cements like iron carbonate (siderite) or iron oxides precipitate within pore spaces around a central organic or inorganic nucleus over millions of years.

Uses & applications

Historically used as a local source of iron ore; occasionally used as decorative garden rocks, landscaping materials, or collected by rockhounds for unique shapes.

Geological facts

Ironstone concretions often weather faster on the outside while maintaining a hard shell, sometimes creating hollow centers known as 'rattle stones' or unusual 'turtle-shell' fractures called septarian nodules.

Field identification & locations

Identified by its distinctive rounded, nodular shape, brownish-red iron staining, and layered or concentric interior structure. Commonly found in shale, sandstone, and coal-bearing sedimentary formations worldwide.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Sedimentary Sandstone

Arenite (Sedimentary Rock)

sedimentary

Fossiliferous Concretion

Septarian Concretion containing Brachiopods/Crinoids

fossil

Banded Siltstone / Mudstone Pebble

Siltstone / Argillite

sedimentary

Shale (or Mudstone/Siltstone)

Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)

sedimentary

Shale

Shale (clastic sedimentary rock)

sedimentary