Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body. "Septaria" comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock. Over time calcite leeched down into the cracks and formed calcite crystals which grew to fill the openings, 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. Septarian is a geode that is a combination of yellow calcite, brown aragonite, grey limestone and white/clear barite, thus it has properties of each of its component minerals.