This huge specimen is one of the largest we’ve seen. Septarian is known to have formed during the Cretaceous Period which began approximately one hundred and forty five million years ago and ended sixty six million years ago around the time the dinosaurs disappeared. They formed in bodies of water as minerals and organic matter accumulated around a centre mass and gradually over millions of years and with the help of various geological processes, hardened and became cemented together forming a nodule.A septarian nodule is composed of the mineral Calcite (yellow centre), aragonite (the brown lines) and limestone (the matrix).