sedimentary
Concretion
Septarian Concretion (Clay-Ironstone Nodules)
Hardness: 3.5-4.0 (Mohs scale), Color: Brown, gray, tan, Luster: Dull or earthy, Structure: Spheroidal or disk-like mass, Cleavage: None (conchoidal fracture).
- Hardness
- 3
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Physical properties
Hardness: 3.5-4.0 (Mohs scale), Color: Brown, gray, tan, Luster: Dull or earthy, Structure: Spheroidal or disk-like mass, Cleavage: None (conchoidal fracture).
Formation & geological history
Formed by the precipitation of mineral cement within the spaces between sediment particles. These often form around a nucleus such as a leaf, shell, or fossil during the Diagenesis stage of sedimentary rock formation, typically millions of years old.
Uses & applications
Used primarily as decorative collector specimens, educational items for geology, and occasionally as garden ornaments or semi-precious stones (when contain septarian patterns).
Geological facts
They are often mistaken for fossils, turtle shells, or dinosaur eggs due to their unusual symmetrical shapes and internal 'veins'. Some of the most famous example are the Moeraki Boulders in New Zealand.
Field identification & locations
Identify by their distinct rounded or lens-like shape which contrasts with the surrounding layered rock. Often found in shale or sandstone beds. Collectors look for pieces that have naturally weathered out of hillsides or riverbanks.
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