Nature possesses a unique control over the formation of CaCO3 crystals that imparts fascinating mechanical properties to many CaCO3-based biomaterials. This high level of control is, at least in part, achieved through the use of certain soluble additives that influence the crystallization of amorphous CaCO3 (ACC). Inspired by nature, excellent work has been performed to elucidate the influence of additives on the crystallization of ACC that is dispersed in bulk aqueous solutions or subjected to elevated temperatures. By contrast, very little is known about the influence of additives on the crystallization of ACC when exposed to a humid environment or elevated pressures. This incomplete understanding restricts the range of properties that can be obtained in synthetic CaCO3-based biomaterials. To address this shortcoming, we study the influence of additives on the humidity- and pressure-induced crystallization of ACC. We find that the humidity-induced crystallization of ACC follows a distinctly different pathway than the pressure-induced one. As a result, the influence of additives on the crystallization kinetics of ACC, and hence, the size, morphology, structure, and orientation of the resulting CaCO3 crystals, differs considerably. These insights offer new opportunities to design CaCO3-based biomaterials whose mechanical properties more closely resemble natural ones.