The influence of mineralogical composition as well as textural characteristics on strength and deformation behaviour of rocks have been investigated in recent years. The results of these studies are based on rocks with simple mineral paragenesis consisting of quartz, feldspar and phyllosilicates. To further investigate the applicability of mineralogical and strength correlations on rocks with more complex mineral paragenesis, gneiss specimens from two locations within the Eastern alps are tested. Those two sample groups have experienced different grades of metamorphism with one developing a complex high-grade metamorphic paragenesis with high garnet and sillimanite contents. Thin section analysis is used to determine the mineralogical composition, texture and grain size. The characterization of strength behaviour is investigated by conducting uniaxial and triaxial compressive tests with ultrasonic wave velocities providing additional information. High-grade metamorphic rocks develop a higher rock strength than low-grade metamorphic rocks. In complex mineral paragenesis, garnet content is found to be the most dominant influencing factor of rock strength. Higher garnet content leads to higher strength in samples with the same phyllosilicate content. Quartz also contributes to higher rock strength, while feldspar and phyllosilicate content do not show a significant correlation with strength. The equivalent quartz content is determined as a simple approach to summarize the mineralogical composition and increases with increasing strength. Textural differences, especially in the formation of phyllosilicate layers, are important for the failure modes and fracture formation. Consistent phyllosilicate layers, as found in high-grade metamorphic rocks, promote shear fractures parallel to the foliation. Even a small deviation from a foliation angle of 90° leads to significant cleavage-fracture formation. Medium-grained augen, which are surrounded by phyllosilicates, are not penetrated by tension cracks. Phyllosilicates are bent around the augen and, therefore, also orientated parallel to the direction of the loading axis. Planar schistosity, as observed in low-grade metamorphic rocks promotes splitting along the foliation whereas deformed garnet porphyroclasts within phyllosilicate layers in high-grade metamorphic rocks prevent this behaviour.