sedimentary
Ironstone Concretion (Boxwork)
Siderite-Limonite Concretion (FeO(OH)·nH2O)
Hardness: 4-5.5 (Mohs scale), Color: Rusty brown, reddish-orange, and black, Luster: Dull to sub-metallic, Structure: Amorphous or microcrystalline, Cleavage: None, Specific Gravity: 3.0-4.3.
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs scale), Color: Rusty brown, reddish-orange, and black, Luster: Dull to sub-metallic, Structure: Amorphous or microcrystalline, Cleavage: None, Specific Gravity: 3.0-4.3.
Formation & geological history
Formed through the precipitation of iron minerals within sedimentary rock layers, often around a nucleus. Differential weathering of softer internal material leaves behind the harder, iron-rich 'boxwork' or 'shell' ridges.
Uses & applications
Primarily used as decorative display specimens, for geological study, and historically as a low-grade iron ore.
Geological facts
These are sometimes mistaken for meteorites or fossils due to their organic-looking, eroded shapes. They often form in sandstone or shale environments during diagenesis millions of years ago.
Field identification & locations
Identified by its high density relative to other sedimentary rocks and its distinctive rusted, scalloped appearance. Commonly found in eroded canyons, riverbeds, and desert regions like the American Southwest.
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