sedimentary

Ironstone Concretion (Limonite/Goethite)

Ferruginous Sandstone / Ironstone (FeO(OH)·nH2O)

Hardness: 4.0-5.5 (Mohs scale); Color: Dark brown, rusted orange, or ochre; Luster: Dull, earthy; Crystal structure: Amorphous or microcrystalline; Cleavage: None/Conchoidal fracture; Specific gravity: 3.3 - 4.3.

Hardness: 4
Color: Dark brown, rusted orange, or ochre
Luster: Dull, earthy

Identified April 12, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of iron-rich minerals (limonite, hematite, or goethite) within sedimentary layers, often around a nucleus like a leaf, shell, or pebble. These occur in oxidized environments and can range from ancient Paleozoic to relatively recent Quaternary ages.

Uses & applications

Historically used as a low-grade iron ore; commonly used as garden stones, decorative landscape gravel, or as specimens for educational geological collections.

Geological facts

Concretions are often mistaken for meteorites due to their heavy weight and dark, weathered 'crust.' Some concretions, known as septarian nodules, can contain beautiful hollow centers filled with crystals.

Field identification & locations

Identify in the field by its high density compared to surrounding rocks, a characteristic rusty-brown streak when rubbed on unglazed porcelain, and often a rounded or orbicular shape. Found globally in creek beds and eroding sandstone cliffs.

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