Calcium carbonate (CaCO3) is one of the most common minerals on Earth; it not only forms rocks like limestone or marble but is also a main component of biominerals such as pearls, the nacre of seashells, and sea-urchin skeletons (1). Despite many years of research, the polymorphism of CaCO3 is still far from being understood. CaCO3 has three anhydrous crystalline forms: calcite, aragonite, and vaterite, with a decreasing thermodynamic stability under aqueous ambient conditions (calcite > aragonite > vaterite) (2). While vaterite is rare in nature, calcite and aragonite are both frequently found in rocks or biominerals (1). A well-known example is the aragonite structure of nacre (3), where the organization of the crystals leads to extraordinary mechanical performance. However, in synthetic systems, crystallization experiments only generate a small fraction of aragonite compared with calcite at ambient conditions and in the absence of additives (4). So, how is the formation of aragonite facilitated in nature, especially in biominerals? In PNAS, Zeng et al. (5) shed light on this matter by showing that aragonite formation is dramatically promoted within confinements.