mineral

Limonite (Ironstone Concretion)

Hydrous ferric oxides (FeO(OH)·nH2O)

Hardness: 4-5.5 on Mohs scale; Color: Dull brown, yellowish-brown, to black; Luster: Dull, earthy, or sub-metallic; Crystal Structure: Amorphous (no visible crystals); Streak: Yellowish-brown

Hardness: 4-5
Color: Dull brown, yellowish-brown, to black
Luster: Dull, earthy, or sub-metallic

Identified May 30, 2026More mineral →

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Physical properties

Hardness: 4-5.5 on Mohs scale; Color: Dull brown, yellowish-brown, to black; Luster: Dull, earthy, or sub-metallic; Crystal Structure: Amorphous (no visible crystals); Streak: Yellowish-brown

Formation & geological history

Formed by the oxidation and hydration of iron-rich sulfide minerals or by the precipitation in marine or marshy mineral-rich environments (bog iron).

Uses & applications

Historically used as a low-grade iron ore and as a pigment for ochre paints. Today, it is primarily a geological specimen.

Geological facts

Limonite is not a single mineral but a mixture of similar hydrated iron oxide minerals, mostly goethite. It is responsible for the yellowish-brown coloring in many sedimentary rocks and soils.

Field identification & locations

Identify by its yellowish-brown streak on a porcelain plate and lack of magnetism compared to magnetite. Commonly found in gossans (weathered tops of ore deposits) or as concretions in sandstone and shale.

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Sandstone (with potential mineral staining/concretions)

Arenite (SiO2 based)

sedimentary

Epidote

Epidote | Ca2(Al2,Fe3+)(SiO4)(Si2O7)O(OH)

metamorphic

Nephrite Jade

Nephrite

Mineral/Rock

Banded Gneiss (River Rock/Pebble)

Gneiss (specifically a banded variety often found as a river rock)

Metamorphic Rock

Biotite Schist

Biotite-rich Schist [K(Mg,Fe)3AlSi3O10(F,OH)2]

metamorphic

Green Apatite on Albite

Fluorapatite [Ca5(PO4)3F] on Albite [Na(AlSi3O8)]

mineral