We report the fabrication, characterization, and a tentative growth mechanism of spherical microparticles with a rough surface fabricated by oxidative aging of ferrous hydroxide. The aging involves the transformation of ferrous hydroxide into Fe3O4 and the growth of the magnetite particles. Scanning electron microscopy, focused ion beam, and transmission electron microscopy studies of the spherical microparticles show that they have a small void in the center and that they are polycrystalline with a typical grain size of 150 nm. The crystallites are oriented along the radial direction of the spheres, stretching themselves from the central cavity to the particle surface, and their crystalline orientations do not keep any obvious relationship. The collected data and the structure suggest that the microparticles' growth mechanism stages: (1) initial nucleation of small magnetite nanoparticles, (2) aggregation to form spherical polycrystalline clusters, (3) direct crystal growth from species in solution, and (4) development of the outer facets. Magnetization measurements are in agreement with the observed crystalline structure. Both X-ray powder diffraction and magnetization measurements indicate that the stoichiometry of these particles is slightly oxidized with respect to Fe3O4.