sedimentary
Ironstone Concretion (Ironstone Gourd)
Limonite/Goethite Concretion (FeOO(H))
Hardness: 4-5.5 (Mohs scale), Color: Rusty brown, orange, to dark ochre, Luster: Dull/Earthy, Crystal Structure: Amorphous or Microcrystalline, specific gravity: 2.7-4.3
- Hardness
- 4-5
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Physical properties
Hardness: 4-5.5 (Mohs scale), Color: Rusty brown, orange, to dark ochre, Luster: Dull/Earthy, Crystal Structure: Amorphous or Microcrystalline, specific gravity: 2.7-4.3
Formation & geological history
Formed through the precipitation of iron oxide minerals within sedimentary rock layers, usually during diagenesis. These are often found in sandstone or shale beds dating from the Cretaceous to the Paleogene periods.
Uses & applications
Primarily of interest to collectors and educators. Historically, high-concentration specimens were used as low-grade iron ore. In some cultures, they are used as decorative 'curiosity' stones.
Geological facts
Also known as 'rattle stones' because the inner core can sometimes weather away, leaving a loose pebble inside that rattles when shaken. They are frequently mistaken for meteorites or fossils due to their organic, bulbous shapes.
Field identification & locations
Identify in the field by the distinct rusty staining on surrounding rock and the 'shell-like' structure that is harder than the host rock. Commonly found in the American Southwest and the Appalachian basins. Look for hollow centers or layered, rinded exteriors.
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