sedimentary
Septarian Nodule or Mud Crack Fossil
Septarian Nodule (Concretion)
Hardness: 3.5-4.0 (for calcite centers); Color: Brown, tan, cream, and grey; Luster: Waxy to dull (earthy); Structure: Massive/Crystalline infilling; Cleavage: None/Conchoidal.
- Hardness
- 3
- Color
- Brown, tan, cream, and grey
- Luster
- Waxy to dull (earthy)
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Physical properties
Hardness: 3.5-4.0 (for calcite centers); Color: Brown, tan, cream, and grey; Luster: Waxy to dull (earthy); Structure: Massive/Crystalline infilling; Cleavage: None/Conchoidal.
Formation & geological history
Formed during the Cretaceous period (50-70 million years ago) from mud balls and organic matter in ancient seabeds. As the mud dried and shrank, cracks formed and were later filled with minerals like calcite and aragonite via groundwater infiltration.
Uses & applications
Used primarily for home decor, lapidary items (bookends, spheres), jewelry, and as educational geological specimens for collectors.
Geological facts
Also known as 'Dragon Stones' due to their scaly appearance. They represent a rare combination of mineral formation and shrinkage-crack patterns that preserve a specific moment in the chemical history of an ancient seafloor.
Field identification & locations
Identify by the angular 'lightning' patterns or cracks separating dark brown mudstone areas from lighter crystalline centers. Commonly found in Utah, Madagascar, and Morocco.
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