sedimentary
Limonite Concretion (Ironstone)
Limonite / Goethite (FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs); Color: Dark brown to yellow-brown or reddish; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7–4.3.
- Hardness
- 4-5
- Color
- Dark brown to yellow-brown or reddish
- Luster
- Dull to earthy
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 4-5.5 (Mohs); Color: Dark brown to yellow-brown or reddish; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7–4.3.
Formation & geological history
Formed by the precipitation of iron oxides/hydroxides from groundwater within sedimentary rock layers, often as nodules or concretions in sandstone or shale. They can form over thousands to millions of years.
Uses & applications
Historically used as a low-grade iron ore. Today, used primarily for educational specimens, aquascaping, and as earth-tone pigments (ochre).
Geological facts
Limonite is not a true mineral but a mixture of hydrated iron oxide minerals. These rounded nodules are sometimes called 'shaman stones' or 'Moqui marbles' in specific regions like the Southwestern USA.
Field identification & locations
Identify in the field by its high density (heavy for its size), rusty streak when scratched, and characteristic pitted or rounded 'nodular' surface. Commonly found in eroded sedimentary basins.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary