Splitting resp. extension fracturing of hard rocks in the neighbourhood of underground openings at distinct depths may be a common phenomenon. Its explanation, however, is insufficient due to the lack of understanding of some causal interdependencies. The splitting cracks occur both on relatively small scales, for example at the walls of a deep borehole or at their bases, and on larger scales, for example around shafts, tunnels or caverns, forming shells at their margins. The phenomenon of splitting crack formation resp. extension fracturing without any kind of shear formation despite overburden pressure conditions is not restricted to brittle hard rocks; it is also related to comparable ductile rock salt. There are different attempts to find theoretical explanations for the rock splitting phenomenon, i.e. the tensile crack formation under general pressure conditions. The diverse theoretical approaches to the splitting process are worthwhile to be scrutinized by experimental element tests using rock samples at extensional as well as compressional triaxial stress conditions. However, there are only rare experimental investigations. In this study three modes of extensional stress conditions in triaxial tests using different boundary conditions and various parameters (loading path, stress or strain rates, porefluid pressure) with diverse types of clastic and igneous rock and comparison with compression triaxial test results were chosen in order to find out relevant factors affecting splitting characterized by tensional crack propagation at a pressure that is effective in the three principle stress conditions. The results partly confirm some theoretical assumptions; nevertheless, they support the experience that tensile cracks in the triaxial extension tests do not occur simply dependent from stress or strain conditions. In a triaxial extensional stress regime tensile crack formation is possible under general pressure conditions even up to moderate minimal principle stresses. Additionally their occurrence depends on time, i.e. the velocities of changes of stress and strain, stress path as well as petrophysical properties like microstructure and moisture.