sedimentary
Septarian Nodule
Septarian Concretion (Calcite, Aragonite, and Limestone)
Hardness varies between 3 and 4 on the Mohs scale due to the calcite and aragonite; colors include brown, yellow, white, and grey; luster is dull to vitreous in the crystalline veins; crystal structure is typically trigonal (calcite) or orthorhombic (aragonite) within the fractures.
Identified More sedimentary →
Explore Septarian Nodule in the encyclopedia →Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness varies between 3 and 4 on the Mohs scale due to the calcite and aragonite; colors include brown, yellow, white, and grey; luster is dull to vitreous in the crystalline veins; crystal structure is typically trigonal (calcite) or orthorhombic (aragonite) within the fractures.
Formation & geological history
Formed in sedimentary environments during the Cretaceous period (approx. 50 to 100 million years ago). Mud-rich sediments accumulated around a fossil or organic core, dried and cracked from shrinking, and groundwater later filled these shrinkage cracks (septaria) with minerals like calcite, aragonite, and sometimes barite.
Uses & applications
Popular in lapidary arts, decorative items, bookends, sphere carvings, and as collectible specimens for mineral enthusiasts.
Geological facts
The name septarian comes from the Latin word 'septum,' meaning partition, referring to the radiating cracks and partitions inside the nodule. When cut and polished, they often resemble turtle shells or alien landscapes.
Field identification & locations
Easily identified by its distinctive rounded, nodular exterior and the spiderweb-like network of inner crystalline cracks visible when cut or broken open. Frequently found in marine sedimentary layers, notably in Madagascar, Utah, and the UK.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Quartz pebble
Silicon dioxide (SiO2)
sedimentary
Fossiliferous Concretion
Septarian Concretion containing Brachiopods/Crinoids
fossil
Generic Pebble / Rock Fragment
Lithic fragment / Weathered rock
sedimentary
Sedimentary Sandstone
Arenite (Sedimentary Rock)
sedimentary
Banded Siltstone / Mudstone Pebble
Siltstone / Argillite
sedimentary