Septarian Concretions or Septarians nodules were formed during the Cretaceous Period around 50-70million years ago. These come from Madagascar. 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. Eventually when the ocean receded, the mud balls dried out and began to shrink and crack into the beautiful patterns that you see inside the nodules. Within these stones you will find a mixture of minerals, commonly Calcite (the yellow parts), Aragonite (the brown lines) and possibly white (Baryte) or grey (Limestone or Chalcedony)... depending on the specific minerals that make up that particular stone.