sedimentary
Sandstone Concretion
Septarian Concretion (Nodule)
Hardness: 3-5 (depending on mineral cement); Color: Grey, tan, earthy; Luster: Dull/Earthy; Structure: Spheroidal, often displaying shrinkage cracks (septaria); Specific Gravity: ~2.5
- Hardness
- 3-5 (depending on mineral cement)
- Color
- Grey, tan, earthy
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-5 (depending on mineral cement); Color: Grey, tan, earthy; Luster: Dull/Earthy; Structure: Spheroidal, often displaying shrinkage cracks (septaria); Specific Gravity: ~2.5
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between sediment grains, often around a nucleus like a leaf or shell. Frequently found in shale and sandstone beds of various geological ages.
Uses & applications
Primarily of interest to geological collectors and educators; larger or more aesthetic septarian nodules are cut and polished for lapidary use and decor.
Geological facts
These are often mistaken for fossils, eggs, or meteorites due to their unnatural symmetry. The cracks visible on the surface suggest it may have a hollow or mineral-filled interior.
Field identification & locations
Identify by its rounded shape, distinct texture compared to surrounding rock, and characteristic 'turtle-back' cracking. Commonly found in eroded creek beds and sedimentary outcrops.
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