sedimentary
Limonite Concretion
Goethite/Limonite (FeO(OH)nH2O)
Hardness 4-5.5, dull/earthy luster, yellowish-brown to dark brown color, amorphous or microcrystalline structure, sub-conchoidal fracture, high density for its size.
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Physical properties
Hardness 4-5.5, dull/earthy luster, yellowish-brown to dark brown color, amorphous or microcrystalline structure, sub-conchoidal fracture, high density for its size.
Formation & geological history
Formed by the precipitation of iron oxide minerals from groundwater within sedimentary rock layers, often as a nodule or concretion around a core. Likely from the Holocene or Pleistocene era.
Uses & applications
Primarily of interest to collectors and used historically as a low-grade iron ore (bog iron).
Geological facts
Commonly known as 'Moqui Marbles' or 'Ironstones' depending on the region. They are often found in large quantities in eroded sandstone beds.
Field identification & locations
Identify by its heavy weight relative to size and the dark brown streak it leaves on unglazed porcelain. Often found in desert environments or riverbeds where sedimentary rocks have eroded.
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