Septarian, correctly known as a septarian nodule or concretion, is distinctive, takes on a high polish, and is classed as a rock, not a mineral. Septarian nodules are made up primarily of three different minerals. The yellow crystals are calcite, the brown lines are aragonite or siderite and the outer shell is limestone.

Despite being quite common, concretions are often considered a geological curiosity because of the many unusual shapes, sizes and compositions in which they occur.

Septarians formed during the Cretaceous Period, which began approximately 145 million years ago and ended 66 million years ago. That's around the time the dinosaurs disappeared. The nodules formed beneath the water as minerals and organic matter accumulated around a center mass. Gradually, over millions of years and with the help of various geological processes, the mass hardened and cemented together to form a nodule.

Septarian nodules may have formed in shallow lakes as the tide caused an accumulated mass to roll gently back and forth. Over time, with the addition of new layers of sticky mud, the size of the mass increased. During the hot summer months, as the water receded, the mud dried out, causing it to crack.

The newly formed structures then became buried under sediment. The cracks slowly filled through seepage with a coarse crystalline substance such as silica (quartz) or calcite from the shells of dead marine creatures.