sedimentary
Ironstone Concretion / Limonite Node
Ferruginous Sedimentary Rock (composed primarily of Limonite FeO(OH)·nH2O and Goethite)
Hardness: 4-5.5 (Mohs scale); Color: Rusty brown, dark reddish-brown, or ocher; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7 to 4.3 (feels heavy for its size due to iron content).
- Hardness
- 4-5
- Color
- Rusty brown, dark reddish-brown, or ocher
- Luster
- Dull to earthy
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Physical properties
Hardness: 4-5.5 (Mohs scale); Color: Rusty brown, dark reddish-brown, or ocher; Luster: Dull to earthy; Crystal Structure: Amorphous or cryptocrystalline; Cleavage: None; Specific Gravity: 2.7 to 4.3 (feels heavy for its size due to iron content).
Formation & geological history
Formed by the precipitation of iron oxides within the pore spaces of sedimentary layers (like sandstone or shale). These often grow around a nucleus of organic material or a pebble. They can form in boggy environments or via groundwater movement through existing strata, ranging from millions to hundreds of millions of years old.
Uses & applications
Historically used as a low-grade iron ore. In modern times, it is used for landscape stone, crushed for road base, or used by lapidaries as a component in 'Moqui Marbles' or 'shaman stones' for metaphysical collections.
Geological facts
Ironstones are responsible for the 'red' in many famous landscapes, such as the Grand Canyon or the rust-colored hills of the Australian Outback. Some concretions are hollow and contain loose sand or rattle when shaken, known as 'rattle stones'.
Field identification & locations
Identify in the field by its distinct rusty appearance, high weight relative to other nearby sedimentary rocks, and often rounded or irregular nodular shapes. It will leave a yellow-brown streak on a porcelain plate. Common in the Southeastern US, UK (Yorkshire), and arid deserts.
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