Septarian

Septarian is a sedimentary concretion formed during the Cretaceous period, approximately 50 to 70 million years ago. It is composed of a combination of calcite, aragonite, and limestone, with cracks or fissures filled with crystallized minerals, often calcite. The name "Septarian" comes from the Latin word "septum," meaning "partition," referring to the internal cracks. These cracks form due to the shrinkage of the mud during the drying process. The appearance is characterized by a brown outer layer (limestone), yellow calcite, and often white or clear calcite crystals filling the cracks. Septarian nodules are found in various locations worldwide, including the United States, Madagascar, and New Zealand.