Septarian Concretion --- Concretions are masses of mineral matter formed when minerals in water are deposited about a nucleus (such as a leaf or shell or other particle) forming a rounded mass whose composition or cement is usually different from the surrounding rock. This can occur at the time of deposition, shortly thereafter, or after the sediment has hardened. Generally, concretions are harder than the rocks around them; therefore, over time the concretions can weather out of the surrounding rocks. Concretions are found world wide, but some of the best come from Wyoming and Kansas. These are formed from any of a number of minerals, including calcite, limonite, barite, pyrite, or silica. They vary widely in shape and size. The smallest are oolites, which can be smaller than the head of a pin. At the other end of the spectrum are the huge spherical Septarian Concretions, the largest of which have diameters of 20 feet or more. The exteriors of septarian concretions are crisscrossed by a network of ridges, giving some of them the appearance of a turtle shell. Geologists think they were formed by the shrinkage of concretions, which caused cracks to form, followed by the deposition of the various such minerals. When the concretions are exposed to weathering, the softer parts between the mineral-filled cracks are eroded and the cracks extend above the surface of the concretion, like ridges or little walls. When cut and polished, they make interesting stones for jewelry, as long as they are well protected. .