Crystallization can be divided into stages primary nucleation is the first. It is the growth of a new crystal. In turn this causes secondary nucleation – the final stage if removal of the crystals is not an issue. Secondary nucleation needs existing crystals to continue crystal growth. In our sugar example, we had got such nuclei when the 'excess' sugar had just about crystallized out. Secondary nucleation is the main stage in crystallization for this is what causes the 'mass production' of crystals.