The resistance of rock to tensile fracture may be measured by its fracture energy GI, which is found to range from 40 to 200 J/m2 in tests on nine types of sedimentary and crystalline rock. Differences in microstructure among the rocks tested are the principal cause of differences in the steady state value of GI, in the distance that a crack must advance before steady state fracturing is attained, and in the amplitude of the fluctuation of GI that accompanies crack advance. When nearly continuous surfaces of weakness are present, as in the Salem limestone, GI is low and attains steady state after only a small amount of crack advance. When a preexisting, interconnected network of microcracks is exploited by the fracture process, GI is large, and steady state is attained only after extended crack propagation. The sensitivity of GI to crack speed and the presence of water is low under the test conditions used in all the rocks examined. However, the magnitude of GI measured in a given type of rock is found to depend on the configuration of the test specimen and on components of stress near the crack tip that do not influence crack growth in linearly elastic materials. The conditions under which GI can be considered a material property are therefore restricted.