sedimentary

Ironstone Concretion

Ironstone (Siderite/Limonite/Goethite)

Hardness: 5-6 (variable); Color: reddish-brown, dark brown, or black; Luster: earthy to sub-metallic; Structure: typically massive or ovoid with internal concentric layering; Specific Gravity: high (~3.0-4.0)

Hardness: 5-6 (variable)
Color: reddish-brown, dark brown, or black
Luster: earthy to sub-metallic

Identified July 4, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of iron-rich mineral cement within the spaces between sediment grains, often around a central nucleus like a fossil or leaf. Common in shale, sandstone, and coal beds from the Paleozoic to Mesozoic eras.

Uses & applications

Historically used as a low-grade iron ore; currently used as decorative garden stones, in landscaping, and as curiosities for collectors.

Geological facts

These are often mistaken for meteorites due to their weight, dark color, and fusion-crust-like weathered exterior. They can sometimes contain perfectly preserved fossils in their core.

Field identification & locations

Identify in the field by their rounded or odd bulbous shapes and dark, rusty appearance contrasting with surrounding rock. Common in the American Midwest and Appalachian regions. Check with a magnet; they are often weakly magnetic.

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Sandstone

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