The complexity of the crystallization pathways leading to a crystalline solid manifest themselves through changes in the solid material as well as in the complementary supernatant. Both pieces of this complementary "pair" need to be considered to fully understand the crystallization process. At the beginning of our investigations, we directly followed the reaction with usual laboratory methods including XRD or pH . We used all those observations to establish a baseline for advanced in-situ methods and pinpoint characteristic moments for ex-situ methods (Table ). During the crystallization of TMPs protons are released (Eqs. 1 & 2), therefore, time-resolved pH measurements provide partial insights into the formation process of these materials. Thus, from simple pH trends, we can identify the characteristic events in the crystallization pathway and indeed confirm the multi-step nature of the mechanism. We further prove with various in-situ and ex-situ methods (Table ) that the multi-step evolution involves transitional amorphous nanophases preceding the formation of the final crystals. Moreover, a directly observable color change of the aqueous solutions/slurries of TMPs also hints towards potential phase evolution and indicates the multi-step character (e.g. transformations liquid-amorphous, liquid-amorphous, amorphous-crystalline). As we demonstrated in a previous work, the color change directly indicates an extensive change in the transition metal coordination environment associated with the transformation of the solids, e.g. from an amorphous to a crystalline state This involves further a structural ordering and reorganization . Even by observing the evolving solutions, one can determine characteristic moments of time where the nature of solids changes, pointing out to a nonclassical nature of the crystallization processes. associated SAED pattern; (C) three dimensional volume rendering from a cryo-ET reconstruction shown over a central XY slice from the reconstruction at t = 300 s, insets in C: The three-dimensional particle surface renderings in selected angles from -45° < 0° < +45° ; a full movie of the reconstruction and additional images can be found in the ESI: Figure . The measurements were supplemented with EDX maps in CTEM, with which we confirmed the presence of N, P, O, and M with M = Ni or Co in the observed aggregates but we also observed their decomposition by degassing of volatile compounds (ESI: Figure ).