Abstract: We propose a thermodynamically consistent phase-field framework for simulating the initiation and evolution of discontinuous structures in geomaterials. The model incorporates new crack driving forces derived from volumetric–deviatoric strain decomposition, while accounting for tension, compression, and shear degradation mechanisms. Inertia effects are included to capture the dynamic development of compaction bands, where wave-like disturbances induce micro-cracking, grain crushing, and frictional rearrangement. A robust monolithic algorithm ensures numerical stability and rapid convergence. The formulation reproduces salient fracture phenomena, including tensile, shear, tensile–shear, and compressive–shear failures, using the Benzeggagh–Kenane criterion. Validation against benchmark problems demonstrates the model’s predictive capability: (i) uniaxial compression tests of rock-like specimens; and (ii) triaxial compression of a V-notched sandstone specimen, consistent with experimental evidence. The proposed framework provides a unified approach to investigate the spatiotemporal evolution of localization and fracture in geomaterials under diverse loading conditions.