sedimentary

Ironstone Concretion

Siderite or Ferruginous Concretion (FeCO3 or Iron Oxides)

AI-generated identification · may be incorrect

Hardness: 3.5-4.5 (Mohs); Color: Rusty brown, reddish, or dark grey; Luster: Dull/Earthy; Crystal Structure: Trigonal (microcrystalline); Specific Gravity: 3.0-4.0

Hardness: 3
Color: Rusty brown, reddish, or dark grey
Luster: Dull/Earthy

Identified March 20, 2026More sedimentary →

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Physical properties

Hardness: 3.5-4.5 (Mohs); Color: Rusty brown, reddish, or dark grey; Luster: Dull/Earthy; Crystal Structure: Trigonal (microcrystalline); Specific Gravity: 3.0-4.0

Formation & geological history

Formed through the precipitation of mineral cement within the spaces between sediment grains (usually sand or mud). They often grow around a nucleus such as a leaf, bone, or shell during diagenesis in sedimentary basins.

Uses & applications

Historically used as a low-grade iron ore; nowadays primarily used for landscaping, educational specimens, or cut open by collectors to reveal potential fossils inside.

Geological facts

Concretions are often mistaken for meteorites, eggs, or cannonballs due to their rounded shapes. The famous Mazon Creek fossils in Illinois are found inside similar ironstone concretions.

Field identification & locations

Identify in the field by their rounded or discoidal shape, heavy weight for their size, and distinct rusty exterior compared to surrounding rock. Common in shale or sandstone cliffs and riverbeds.