Amorphous calcium carbonate (ACC) is a precursor to a wide range of biominerals. It is found highly hydrated in calcified tissues of marine organisms, and computational simulations indeed suggest that its stability is regulated by the presence of water in its structure1. In fact, its stability is controlled by the presence of Mg2+ cations in the structure and to a rigid network of H-bonds that prevents its crystallization. The present study is aimed at probing the link between this persistence to crystallization and the water self-diffusion dynamics using the QENS technique.