Septarians were formed during the Cretaceous Period, around 50 to 70 million years ago. Sea levels were much higher then and the Gulf of Mexico reached inland to Southern Utah where many of the septarian nodules are found. They are also found in Madagascar where conditions were similar.
Periodic volcanic eruptions killed the smaller sea life which sank to the sea bed and started decomposing. The minerals in the shells and carcasses attracted sea floor sediments which accumulated around the carcasses and formed nodules or mud balls. When the ocean eventually receded, the mud balls dried out and began to shrink and crack into the beautiful patterns that you see inside the septarian nodules.
Over the eons, calcite leeched down into the cracks and formed calcite crystals which grew to fill the cracks. The interface between the calcite and the bentonite clay transformed into aragonite which is the dark brown crystal layer. The bentonite mud was eventually replaced with limestone which completed the transformation of the entire nodule to stone.
Length - 11.1 cm
Width - 12.3 cm
Height - 7.2 cm