sedimentary

Limonite (Ironstone Concretion)

Limonite (FeO(OH)·nH2O)

Hardness: 4 to 5.5 on Mohs scale; Color: yellowish-brown to dark brown or blackish; Luster: earthy to sub-metallic; Crystal structure: Amorphous/Microcrystalline; Cleavage: none; Specific Gravity: 2.7 to 4.3.

Hardness: 4 to 5
Color: yellowish-brown to dark brown or blackish
Luster: earthy to sub-metallic

Identified July 11, 2026More sedimentary →

Explore Limonite (Ironstone Concretion) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms as a secondary mineral through the oxidation and hydration of iron-rich minerals (like pyrite or magnetite) or by precipitation from iron-rich groundwater. Found in sedimentary deposits and weathered rock zones (gossans).

Uses & applications

Historically used as an iron ore and as a pigment for paints (brown ochre). It is mostly collected as a mineral specimen today, especially when forming unique botryoidal or concretionary shapes.

Geological facts

Limonite is not a singular mineral but a mixture of hydrated iron oxides, primarily goethite. It was one of the earliest pigments used by humans, appearing in Neolithic cave paintings.

Field identification & locations

Identified by its yellow-brown streak (when rubbed on unglazed porcelain) and its relative weight compared to non-metallic rocks. Common in boggy environments and weathered soil profiles. Collectors look for unique 'moqui marbles' or iridescent surfaces.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Mudstone

Argillite / Siliciclastic sedimentary rock

sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Brown Sandstone (River Rock)

Arenite

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary