Septarian concretions or septarian nodules, are concretions containing angular cavities or cracks, called "septaria". The word comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock.
Septarian dragon eggs are Septarian nodules that have been carved and polished into a egg shape. ... Septarian geodes are a specific geological formation, that occur when minerals (primarily calcite and aragonite) fill in cracks in a concretion. This is a particularly nice one.