Glass samples in the GeO2 center dot SiO2 center dot Nb2O5 center dot K2O system, with different K2O/Nb2O5 ratios, were prepared, and glass-ceramics were obtained by controlled crystallization. Characterization included density measurements (Archimedes' method), differential thermal analysis, X-ray diffraction (XRD), and Raman and UV-Visible spectroscopies. Crystal volume fractions were estimated from density measurements, while XRD was used to estimate the crystal size, by the Scherrer formula. XRD and Raman data showed that a K2O/Nb2O5 ratio of 1 promoted the formation of the crystalline phase K3.8Nb5Ge3O20.4 and transparent glass-ceramics could be obtained with a crystalline fraction of 79 % and an average crystal size of 19 nm, while a K2O/Nb2O5 ratio of 2 promoted the formation of KNbO3 crystalline phase, and a translucent glass-ceramic was obtained with a crystalline fraction of 27 % and a crystal size of 14 nm. The Maker fringe method was used to compare the second-harmonic generation efficiency of the glass-ceramics with a Y-cut quartz reference.