The synthesis of monodispersed, amorphous cobalt nanoparticles coated with silica in aqueous/ethanolic solution is described. Both the core size and the silica shell thickness can be controlled through the synthetic conditions. Furthermore, the transformation of the cobalt cores into crystalline, metallic cobalt upon annealing in air is demonstrated through X-ray diffraction data. Both the initial, amorphous nanoparticles and their crystalline counterpart are magnetic, which promises important applications for ferrofluid preparation and for magnetic recording media.