Septaria ball on black base

A septaria of 120 million years old, from Madagascar. It has been formed on layers rich in clay and it started with an organic remain (animal or vegetal) and the nodules were formed around it, with the wave agitation. But some anaerobic bacterias (able to live without oxygen, but producing ammonia), were trapped inside the nodule and kept developping. But ammonia, when in contact with sea water caused the precipitation of sulfates and carbonates. With time, the coats of clay become thicker and caused the suffocating of bacterias, and end of the nodule's growth. The carbonates, sulfates and sea water were saturated with Hydrogen sulfide and allowed shrinkage craks to form (that's a geode!) Later they have been filled by the black calcite.