Septarians are a unique combination of Brown Aragonite which has a rhombic crystal system, Calcite that has a trigonal crystal system, some limestone, and some clear to white Barite. It was formed between 50 to 70 million years ago as a result of volcanic eruptions. The tumbled variety I carry has come from Madagascar.
Decomposing sea life that had been killed by volcanic eruptions was attracted to the sediment around them and formed mud balls. As the ocean receded, the balls were exposed to air and sun which caused them to dry and crack. They also shrank at the same time trapping the cracks inside. As decomposed Calcite from the shells was carried down into the cracks in the mud balls, Calcite crystals formed. This thin wall of Calcite then transformed into Aragonite separating the heavy clay exteriors from the Calcite centers.