sedimentary
Ironstone Concretion
Siderite or Limonite Concretion
Hardness: 4-5.5 on Mohs scale; Color: Brown, yellowish-brown, to rusty black; Luster: Dull to earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.0-3.9.
- Hardness
- 4-5
- Color
- Brown, yellowish-brown, to rusty black
- Luster
- Dull to earthy
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 4-5.5 on Mohs scale; Color: Brown, yellowish-brown, to rusty black; Luster: Dull to earthy; Crystal Structure: Amorphous or microcrystalline; Cleavage: None; Specific Gravity: 3.0-3.9.
Formation & geological history
Formed by the precipitation of iron-rich minerals (like siderite, hematite, or limonite) around a nucleus in sedimentary rock layers. This often occurs shortly after sediment deposition in marsh, river, or coastal environments.
Uses & applications
Historical source of low-grade iron ore; used as decorative garden stones; often collected by hobbyists for their odd, 'alien-egg' shapes.
Geological facts
Concretions are often mistaken for meteorites or dinosaur eggs due to their heavy weight and rounded shapes. They sometimes hold well-preserved fossils at their center.
Field identification & locations
Identify by its rounded, nodular shape, high density (it feels heavy for its size), and rusty appearance. Commonly found in eroded creek beds or sandstone/shale formations.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary