sedimentary

Limonite Geode (Ironstone Concretion)

FeO(OH)·nH2O (Hydrated Iron Oxide)

AI-generated identification · may be incorrect

Hardness: 4.0-5.5; Color: Dark brown, yellowish-brown, to black; Luster: Dull to sub-metallic; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Hardness: 4
Color: Dark brown, yellowish-brown, to black
Luster: Dull to sub-metallic

Identified June 25, 2026More sedimentary →

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Physical properties

Hardness: 4.0-5.5; Color: Dark brown, yellowish-brown, to black; Luster: Dull to sub-metallic; Crystal Structure: Amorphous; Cleavage: None; Specific Gravity: 2.7-4.3

Formation & geological history

Formed by the precipitation of iron minerals from groundwater within sedimentary rock layers, often as nodules or concretions in sandstone or shale. They can be millions of years old, dating back to various geological periods like the Cretaceous.

Uses & applications

Primarily used as a minor ore of iron, historic yellow-ochre pigment, and as a collector's specimen. In some cultures, they are used as lucky stones or talismans.

Geological facts

Limonite is not a true mineral but a mixture of hydrated iron oxide minerals. It was historically used as a source for iron smelting and is the primary pigment in 'yellow ochre'.

Field identification & locations

Identify by its rusty brown color, high density compared to surrounding rock, and often irregular, lumpy exterior. Commonly found in stream beds or weathered sedimentary outcroppings.