sedimentary

Ironstone Concretion

Siderite or Hematite-rich Concretion (FeCO3 / Fe2O3)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 (Mohs), Color: Dark brown to reddish-brown, Luster: Dull or sub-metallic, Crystal structure: Microcrystalline, Cleavage: None/Conchoidal fracture, Specific gravity: 3.5-4.5.

Hardness: 4

Identified June 28, 2026More sedimentary →

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Physical properties

Hardness: 4.0-5.5 (Mohs), Color: Dark brown to reddish-brown, Luster: Dull or sub-metallic, Crystal structure: Microcrystalline, Cleavage: None/Conchoidal fracture, Specific gravity: 3.5-4.5.

Formation & geological history

Formed by the precipitation of mineral cement (iron oxides or carbonates) within the spaces between sediment particles, often around a nucleus like a leaf, shell, or bone. Common in shale or sandstone layers.

Uses & applications

Primarily of interest to collectors or as decorative items. Historically, some ironstone nodules were used as a minor iron ore.

Geological facts

These are often mistaken for meteorites or fossils due to their unusual shapes and high density. They are known as 'Moqui Marbles' if they are spherical and found in the American Southwest.

Field identification & locations

Identify by its heavy weight relative to size and smooth, weathered surface with possible shrinkage cracks. Common in desert environments and sedimentary basins.