The effect of the addition of NiO on the crystallization mechanisms has been studied for SiO2–Al2O3–MgO glass-ceramics. A detailed Rietveld analysis has been made to determine the chemical composition and the cation distribution in the spinel phase and the evolution of Ni2+ coordination site using diffuse reflectance spectroscopy. Powder X-ray diffraction (XRD) results show the formation of β-quartz and spinel and the nickel fraction in the spinel phase decreasing as the plateau temperature increases. The determination of cation distribution in spinel using XRD reveals a preference of Ni2+ and Al3+ for octahedral sites and an increasing proportion of Ni2+ in tetrahedral sites for heat treatments at high temperature (1100°C–1200°C). Diffuse reflectance spectroscopy shows that Ni2+ is mainly fivefold coordinated in the starting glass, playing an important role in the crystallization of the spinel phase and it confirms the presence of tetrahedral and octahedral Ni2+ in the glass-ceramics. The combination of these two techniques also indicates a chemical heterogeneity of spinel crystals with Ni2+-rich and Mg2+-rich zones.