Truly spherical silver nanoparticles are of great importance for fundamental studies including plasmonic applications, but the direct synthesis in aqueous media is not feasible. Using the commonly employed copper-based etching processes, isotropicplasmonic response can be achieved by etching well-defined silver nanocubes. Whilst spherical like shape is typically prevailing in such processes, we established that there is a preferential growth towards silver rhombicuboctahedra (AgRCOs), which is thethermodynamically most stable product of this synthesis. The rhombicuboctahedral morphology is further evidenced by comprehensive characterization with small-angle X-ray scattering in combination with TEM tomographyand high resolution TEM. Wealso elucidate the complete reaction mechanism based on UV-Vis kinetic studies, and the postulated mechanism can also be extended to all copper-based etching processes.