sedimentary

Limonite Concretion

Limonite (Hydrated iron(III) oxide-hydroxides, FeO(OH)·nH2O)

AI-generated identification · may be incorrect

Hardness: 4-5.5; Color: Dark brown to yellowish-brown; Luster: Dull or earthy; Crystal Structure: Amorphous; Streak: Yellow-brown; Specific Gravity: 2.7-4.3.

Hardness: 4-5
Color: Dark brown to yellowish-brown
Luster: Dull or earthy

Identified June 19, 2026More sedimentary →

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Physical properties

Hardness: 4-5.5; Color: Dark brown to yellowish-brown; Luster: Dull or earthy; Crystal Structure: Amorphous; Streak: Yellow-brown; Specific Gravity: 2.7-4.3.

Formation & geological history

Formed by the chemical precipitation of iron-rich fluids within sedimentary rock layers, often as groundwater moves through sand or clay environments. They can form at various geological ages and often grow concentrically around a nucleus.

Uses & applications

Historically used as a low-grade iron ore and as a pigment for ochre. Today, they are primarily of interest to collectors and hobbyists.

Geological facts

Limonite is not a true mineral but a mixture of similar hydrated iron oxide minerals. Concretions like this are often mistaken for meteorites or fossils due to their unusual shapes.

Field identification & locations

Identify in the field by its characteristic yellow-brown streak, heavy weight for its size, and common occurrence in sandstone or shale beds. Often found in streambeds where the harder concretion has eroded out of softer rock.