Polycrystalline SiGe layers have been oxidized in either dry or wet atmospheres in order to form Ge nanoparticles embedded in a dielectric matrix. The evolution of the growing oxides and the SiGe layer during the oxidation processes have been characterized using Raman spectroscopy, X-ray diffraction, Rutherford backscattering and Fourier transform infrared spectroscopy. Ge nanocrystals have been formed in both oxidation atmospheres. Violet luminescence emission (3.1 eV) has been observed and its relation to the oxidation processes has been studied. For dry oxidation, the luminescence intensity appears suddenly when the pure segregated Ge layer starts to be oxidized forming Ge nanocrystals. It remains as long as Ge nanoparticles are present. For wet oxidation, an initial luminescence appears, that depends on the oxide thickness, which is related to the formation of Ge-rich nanoclusters trapped in the SiGeO growing oxide. A sharp increase of the luminescence for long oxidation times is then observed, which is related to the formation of Ge- nanoparticles by the oxidation of the segregated Ge. In both processes the luminescence is quenched for long enough oxidation time. The intensity of the luminescence in the dry oxidized samples, for equal initial thickness of the polycrystalline SiGe layer, is 10 times higher than in the wet oxidized ones. The violet luminescence is neither related to the recombination of excitons inside the Ge nanocrystals nor to defects in the germanium oxide. Ge oxygen deficient centers, located at the interface between the nanoparticles and the dielectric matrix, are proposed as the origin of the violet luminescence.