A study of bulk second harmonic generation (SHG) response of lithium niobium silicate glass-ceramics is presented. The observed macroscopic SHG signals have an isotropic 3D nature. To interpret this particular nonlinear optical response, a multiscale approach is used in which clear correlations between structure and optical response are characterized from the sub-micrometer to the millimeter scale. In particular, it is inferred that the radial distribution of the LiNbO3 crystallites in spherulite domains is at the origin of the isotropic bulk second order optical property. It is suggested that spherulitic crystallization in glass-ceramic is a challenging method to elaborate isotropic nonlinear optical properties in inorganic materials.