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Calcium carbonate (CaCO3) is ubiquitous due to its presence in several biomineralized entities, and it often serves as a model system to study crystal nucleation and growth. CaCO3 exists in three crystalline polymorphs, namely, calcite, vaterite, and aragonite—all of which can be precipitated in the laboratory under suitable reaction conditions, e.g., temperature and concentration. The crystallization of the different polymorphs of CaCO3 can be affected by additives that inhibit or promote the formation of specific polymorphs or alter their growth and morphology. The role of one such additive, fetuin-A, a plasma carrier protein in the animal bloodstream, on the structure and morphology of CaCO3 microparticles was investigated in this article. We have utilized coherent x-ray diffraction imaging in combination with wide-angle x-ray diffraction and transmission electron microscopy to visualize the 3D morphology of CaCO3 grown under well-defined experimental conditions. The resulting CaCO3 particles crystallized in the presence of fetuin-A were primarily composed of calcite that showed unidirectional faceted growth along the (104) planes, along with minor vaterite inclusion and internal cavities containing amorphous CaCO3 (ACC). Our observation of a single particle containing ACC, vaterite, and calcite provides direct evidence of the full crystallization pathway of CaCO3 with nanoscale spatial resolution, capturing the transformation from metastable ACC to the thermodynamically stable phase, calcite.