The name “septarian nodules” is derived from the Latin word “septum,” meaning seven, in reference to partitions dividing cavities. More specifically, the name refers to carbonate-rich nodules having an internal structure comprised of a series of carbonate-filled cracks or cavities, which separate polygonal blocks of hardened sedimentary material, such as mudstone.
The exterior of a septarian nodule, 13 cm tall and 11 cm wide. Utah.
The interior of the same nodule, shown above. The interior of a septarian nodule can be, however, very interesting and eye-catching.
The nodules usually form early in the burial history of the muddy sediment before the rest of the sediment hardens into rock. Afterward, compaction causes cracks/cavities to form, and groundwater with dissolved minerals infills the cracks/cavities. Upon evaporation, the minerals crystallize out of the water.
Septarian nodules, more accurately referred to as septarian concretions, have a smooth rounded exterior, which is normally gray. The mineral-filled cracks in the interior are typically yellow (calcite), but some also can be partially brown (aragonite = another carbonate mineral).
This septarian nodule (12 cm wide and 15 cm tall) was cut (slabbed) in order to show the intricacy of the infilled cavities.
Lastly, I included this slabbed (cut) septarian nodule because it is a nice specimen and one that was used in one of my previous posts (July 17, 2014) entitled "Pseudofossils."The rounded and smooth exterior of most of the septarian nodules have been mistakenly by some collectors as “dinosaur eggs,” which they are NOT! Pseudofossils are inorganic objects.
Rock hounds commonly cut the septarian nodules in half and polish them. They can sell for low to high prices, depending on the complexity of the radiating crack pattern and the variety of the coloration.