What is a septaria in geology ?

Septaria : definition

A septaria is a generally calcareous nodule, located in clay layers, affected by shrinkage cracks which have created a central void partially filled by crystals of calcite, gypsum, quartz, barite, etc... Polished septaria have sometimes a decorative value.

Photo : barite in septaria from Col de Pré Guittard, Drôme, France