<p>Coal consists of a matrix intersected by a multi-scale cleat system, often partially or fully filled with calcite to form mineral veins. How such veins influence failure remains insufficiently constrained. This study integrates laboratory observations and finite element modeling to quantify the mechanical role of calcite veins and their contact conditions in veined coal. Uniaxial compression tests on calcite-veined bituminous coal were monitored using high-definition video, strain measurements, and acoustic emission (AE). Results show a consistency of macro-fracture development, stress drops and AE activities, producing a mixed shear–tensile failure mode characterized by central shear localization with en-echelon tensile cracking. Microscopic observations reveal that long, sparse distributed calcite veins with pre-existing microcracked interfaces preferentially guide fracture development, while densely mineralized zones and intact grains locally impede crack propagation. Three-dimensional micro-CT statistics were used to construct representative two-dimensional FEM models under linear elasticity. Simulations show that calcite veins amplify local stresses, and that microcracked vein–matrix interface further intensifies stress localization and promote mixed-mode failure. Compared with cleats alone, vein–microcrack interactions increase tensile failure potential and introduce additional shear risks. These findings underscore the critical role of vein–matrix contact conditions in controlling both microscopic fracture processes and macroscopic failure modes, providing mechanistic insight into instability in calcite-veined coal and emphasizing the importance of explicitly considering interface conditions in coal seam stability assessment.</p>