The exploitation of deep-earth energy resources demands rock excavation technologies that are efficient, compact, and reservoir-friendly. While high-pressure abrasive water jets (HPAWJ) are effective, they suffer from heavy logistical burdens and potential formation damage. This study investigates the feasibility and energy economy of an abrasive gas jet (AGJ) system driven by a Laval nozzle to break hard rocks using standard industrial low-pressure air (<0.6 MPa). Systematic experiments were conducted on sandstone, limestone, and granite to determine critical breaking pressure thresholds and characterize the evolution of hole depth, diameter, and volumetric removal under varying jet pressures and exposure times. The results quantify distinct thresholds of ≤0.05 MPa for sandstone, 0.15 MPa for limestone, and 0.25 MPa for granite, confirming that dynamic tensile fracture enables hard rock erosion without ultra-high-pressure intensification. Lithology-dependent erosion kinetics were elucidated: sandstone exhibits a non-linear saturation trend due to the debris cushioning effect; limestone maintains stable linear growth via lamellar spalling; while granite is dominated by brittle chipping but faces deceleration in deep cavities due to geometric confinement. In terms of energy economy, based on the rigorous adiabatic compression work model, the AGJ system achieves a specific energy of 4.43 × 1010 J/m3 for fracturing granite at 0.25 MPa. This represents an energy cost reduction of approximately 39% compared to submerged abrasive water jets operating at 25 MPa, demonstrating that high-velocity compressible flow can match hydraulic breaking efficiency at 1/100th of the driving pressure. These findings establish a thermodynamic justification for utilizing low-pressure AGJ and propose lithology-specific operational strategies for subsurface energy engineering.