Shear fracture energy has been measured for a granite and a limestone using a torsional method. We monitor failure with periodic stiffness determinations and measure strain energy released by integrating the load-displacement record. Values of strain energy release rate (G3C) obtained for zero normal stress lie in the range 102-203 Jm-2. These fall between the values of G1C from tensile experiments and of shear fracture energy measured under high normal stress in triaxial tests.