New 30 GeO2-30 TeO2-15 ZnO-10 Na2CO3-10 CaF2-3 La2O3-x ErF3 (x = 2, 4) glasses have been synthesized and submitted to varied heat treatments to provoke nanocrystallization in their matrix. The Er3+-doped materials containing typically 50 Å-sized nanocrystallites have been characterized by DSC, ultrapicnometry, XRD, HRTEM, EDS, SAED, optical spectroscopy and VSM techniques. A Judd-Ofelt analysis could be performed from the absorption spectra and led to refined intensity parameters. The local structural changes around Er3+ cations were mostly evidenced by important modifications of the hypersensitive optical absorption bands (4I15/2 → 2H11/2, 4I15/2 → 4G11/2) and lesser modifications of the emission cross sections and experimental lifetimes. Besides, cycling around the crystallization temperature with a VSM showed some kind of “crossover” between the Curie-Weiss behavior of the Er3+ cations dissolved in the glass and those dissolved in the nanocrystallites, with a striking change of magnetic interactions sign from anti- to ferromagnetic. The crystal field Hamiltonian could be refined in C2 symmetry, thanks to the emission and excitation spectra recorded at 80 K, and the crystal field strength calculated.