The magnetic properties of artificial opals infiltrated with nickel nanoparticles are studied both experimentally and theoretically. The response of the composite has a dominating ferromagnetic behavior at low magnitude fields (below the saturation region), then it is followed by a dominating diamagnetic one as the intensity of the external applied field is increased. These characteristics are not observed in most cases due to the low magnetic response of the elements involved. Diamagnetism in the artificially synthesized opals was observed, having a negative volume magnetic susceptibility of the order of 10⁻⁵. Also theoretical calculations were performed on the basis of the Landau-Lifshitz-Gilbert (LLG) equation, taking into account the diamagnetic response of the host material. Results obtained show the importance of the local magnetic field on the ferromagnetic nanoparticles, as well as the importance of the nature of the magnetic response of SiO₂ spheres, which aligns in the opposite direction of the external applied field.