Ironstone Concretion (Nutstone)

Physical Properties

Hardness: 5-6 (Mohs), Color: Dark brown to reddish-orange, Luster: Dull/Earthy, Crystal Structure: Amorphous or Fine-grained, Specific Gravity: 3.0-4.0

Formation & Geological History

Formed by the precipitation of mineral cement (iron oxides) within the spaces between sediment grains, often during the diagenesis of sedimentary rocks in bog or lake environments.

Uses & Applications

Historically used as an low-grade iron ore; used today in landscaping and as minor curiosities for rock collectors.

Geological Facts

These are sometimes called 'nut stones' or 'omphalos stones' due to their rounded, often pitted shapes. They can sometimes contain a core of a different mineral or even a fossil.

Field Identification & Locations

Identify by its unusually high weight (density) for its size and its dark rusty color. Often found in areas where sandstone or shale has weathered away.

Identified on: 4/29/2026

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