Septarian nodules are round or ovoid concretions, compact bodies of certain minerals formed by accretion (agglomeration) within a rock. The nodules or concretions most frequently consist of calcite, claystone or rhyolite, a volcanic deposit. Septarian nodules have an interior structure formed by irregular cracks that result from drying up and shrinkage. The cracks and voids may (or may not) later be filled up by precipitation of some other mineral like agate or calcite. Cut in half and polished, these nodules display fascinating configurations, like seen in this example from Antwerp, Belgium Actual diameter is 44 cm.