We present the magnetic properties of magnetic glass ceramics obtained by crystallization of Fe containing borosilicate glass. Two types of nucleators have been used: Cr₂O₃ and P₂O₅. The role of the nucleators proved to be crucial in the size and morphology of the crystallites developed within glassy matrix as well in the magnetic response. The former stimulates the growth of regular single crystals uniformly dispersed within the matrix whereas the latter leads to the formation of grains made of tiny (30 nm), nanocrystals. The magnetic response depends on the amount of Fe ions left dispersed within glassy matrix as paramagnetic ions. Although P₂O₅ leads to the best structural magnetite, almost 42% of Fe ions are left dispersed in the matrix without magnetic interaction. In the case of Cr₂O₃, the paramagnetic Fe is decreased to 12% but structural deficiency in the occupancy of the Fe sites of magnetite is revealed by Mössbauer spectroscopy.