We report on the formation of a transparent magnetic material containing nanocrystalline face-centered cubic (FCC) Ni produced by selective oxidation of Ni62Nb38 glassy alloy. Our results demonstrate the formation mechanism and provide the detailed structural characterization of the samples obtained. The proposed oxidation de-alloying method provides a simple way of producing samples containing fine nanocrystalline FCC Ni (average size ~ 3 nm) particles homogeneously distributed in an amorphous Nb2O5 matrix which is applicable as a magneto-optic material.