Banded skarns are metasomatic rocks produced by oscillatory precipitation on a centimetre scale. These rocks result from the transformation of homogeneous carbonate materials by aqueous fluids under metamorphic conditions. After reviewing ordinary, unbanded zoning in skarns and the evolution of this zoning, descriptive elements of banded skarns are identified. It is proposed that banded skarns occur mainly at the rare moment when different minerals (or groups of minerals) compete for the same zone of the metasomatic column. A qualitative preliminary model explaining the appearance of oscillatory precipitations is outlined. Banded skarns are an example of geochemical dissipative structure, showing the onset of self-organization because of the interplay of competitive phenomena inside the system, and not because of external template.