sedimentary
Iron Concretion
Ferruginous concretion (Fe2O3·nH2O / FeO(OH))
Hardness: 4-6 (variable), Color: Rusty orange, reddish-brown to dark gray/black, Luster: Dull or earthy, Structure: Concretionary or botryoidal, Cleavage: None, Specific Gravity: 3.0-4.5 (heavy for its size)
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Physical properties
Hardness: 4-6 (variable), Color: Rusty orange, reddish-brown to dark gray/black, Luster: Dull or earthy, Structure: Concretionary or botryoidal, Cleavage: None, Specific Gravity: 3.0-4.5 (heavy for its size)
Formation & geological history
Formed by the precipitation of mineral cement, typically iron oxides like limonite or goethite, within the spaces between sediment grains in sedimentary rocks over millions of years
Uses & applications
Predominantly used as a minor iron ore in the past; today, they are primarily collected as geological curiosities or used in educational displays
Geological facts
These are often mistaken for meteorites or fossils due to their unusual shapes and high density. In some regions, large versions are known as 'Moqui Marbles' or 'Septarian nodules'
Field identification & locations
Look for a heavy rock with a rusty exterior and a layered or concentric interior. They are commonly found in sandstone or shale formations and are frequently unearthed during construction or erosion
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