sedimentary

Ironstone Concretion

Goethite-rich Siltstone / Siderite (FeCO3)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 (Mohs), Color: Rusty brown, yellowish-orange to dark grey, Luster: Dull/earthy/metallic, Crystal Structure: Amorphous or Cryptocrystalline, cleavage: none

Hardness: 4

Identified May 26, 2026More sedimentary →

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Physical properties

Hardness: 4.0-5.5 (Mohs), Color: Rusty brown, yellowish-orange to dark grey, Luster: Dull/earthy/metallic, Crystal Structure: Amorphous or Cryptocrystalline, cleavage: none

Formation & geological history

Formed by the precipitation of mineral cement within the spaces between sediment grains. This often occurs in claystones or siltstones when iron-rich fluids react with organic matter or groundwater during diagenesis.

Uses & applications

Historically used as a low-grade iron ore; commonly collected as geological curiosities due to their unique shapes (sometimes resembling fossils).

Geological facts

Ironstones can sometimes form 'Septarian nodules' with internal cracks filled with calcite. They are often mistaken for meteorites or dinosaur eggs by amateur collectors due to their weight and shape.

Field identification & locations

Identify in the field by its high density relative to surrounding rocks and its rusty, oxidized outer crust. Commonly found in eroded creek beds or shale outcrops. Use a magnet to check for high iron content.