sedimentary

Ironstone Concretion / Limonite Node

Limonite (Hydrated Iron Oxide, FeO(OH)·nH2O) with Goethite/Hematite

AI-generated identification · may be incorrect

Hardness: 4.0-5.5 on Mohs scale. Color: Dull brown, ochre-yellow, or rusty-orange. Luster: Earthy to sub-metallic. Crystal structure: Amorphous to microcrystalline. Cleavage: None. Specific gravity: 2.7-4.3.

Hardness: 4
Color: Dull brown, ochre-yellow, or rusty-orange
Luster: Earthy to sub-metallic

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of iron-rich minerals in the spaces between grains of sedimentary rocks (like sandstone or shale). Usually forms in oxygen-rich groundwater environments or bog deposits. Age varies widely from millions to hundreds of millions of years old.

Uses & applications

Historically used as a low-grade iron ore. Today, it is primarily used as a pigment (ochre) for paints and dyes. In landscaping, it is used as decorative stone. Collectors value unusual shapes.

Geological facts

Ironstone concretions can sometimes form 'rattle stones' (Klapperstein) if the inner core shrinks and becomes loose within the harder outer shell. They are often mistaken for meteorites due to their heavy weight and rusty exterior.

Field identification & locations

Identify by its high density relative to other rocks, a 'rusty' streak (yellow-brown) when rubbed on unglazed porcelain, and lack of magnetism (unless it contains magnetite). Common in riverbeds, clay pits, and eroding cliffs.