<p>Understanding the fracture toughness of weathered selenite is critical for assessing the stability of stone cultural heritage under environmental degradation. The three-point bending test is a key method for evaluating the mode I fracture toughness in such brittle materials. This work investigates the influence of weathering degree on the mechanical response, nonlinear deformation behavior, and acoustic emission (AE) characteristics of selenite rock under mode I loading conditions by three-point bending test. A three-dimensional grain-based model (3D-GBM), incorporating 3D Voronoi tessellation and a flat-joint contact model with unbonded contacts, was employed to simulate the heterogeneous crystalline microstructure and pre-existing microcracks typical of weathered selenite. Slightly weathered selenite samples collected from the Garisenda Tower in Bologna were utilized for microparameter calibration. Numerical models representing moderately and highly weathered conditions were developed by systematically increasing microcrack density and width. Numerical results demonstrate that increased weathering intensifies the nonlinear deformation behavior, manifested by more pronounced strain-softening and strain-hardening phases, larger failure displacements, and a marked reduction in mode I fracture toughness from 0.181 to 0.052&#xa0;MPa·m^1/2. Force chain analyses reveal a transition toward more heterogeneous stress transmission in highly weathered samples, where load-bearing is concentrated on fewer contacts, thereby promoting dispersed microcrack initiation and coalescence. AE analysis indicates that, with increasing weathering, the spatial distribution of AE events evolves from a localized fracture plane to a more diffuse and random pattern. Concurrently, the maximum AE magnitude decreases, and the <i>b</i>-value increases from 2.01 in slightly weathered samples to 3.22 in highly weathered ones, reflecting a shift from brittle to ductile failure mechanisms. A sharp decline in the<i> b</i>-value is observed near peak loading, serving as a potential precursor to impending catastrophic failure in weathered selenite. This work underscores the necessity of capturing microstructural heterogeneity and progressive damage processes to better understand weathering-induced degradation in crystalline rocks. The combined application of micromechanical modeling and AE monitoring provides a robust framework for evaluating and preserving stone cultural heritage materials subjected to natural weathering.</p>