The combination of natural processes giving rise to the formation of the carbonate-rich septaria remains unclear, but likely involves microbial activity and oxidation of the organic matter in the clay sediment as an internal source of carbonates. The calcium ions present in seawater, or in the pore water of the surrounding clay sediments, slowly diffuse towards the center of the initially soft concretion and progressively precipitate in contact with the carbonate anions present along their path and produced by the decaying organic matter. Indeed, when the pore water of the clay sediment becomes locally saturated with respect to calcium carbonate, this latter precipitates and progressively starts to cement the porosity of the initial medium (decaying marine organisms?). The cementation process seems to proceed from outside (harder) to inside (softer) along with the Ca2+ ions diffusion transport.