Micro-textural variations are essential for understanding variation in strength and failure behavior of rock material. Therefore the effects of petrography and mineralogy on geomechanical properties have been widely investigated. In this study andesitic rock samples with different micro-textural characteristics from three different locations in Turkey were selected. Unconfined compressive strength tests revealed substantial variations in Unconfined Compressive Strength (UCS) and the micro-crack initiation (sigma(CI)) threshold. Detailed petrophysical, petrographic and mineralogical analysis (XRD) were used to understand these strength variations. The synthesis of strength, petrographic, micro-structural and mineralogical data suggest, that both, the peak strength and crack initiation threshold are strongly influenced by the distribution of plagioclase, amphibole and biotite phenocrystals, and the mineral composition of the fine-grained matrix.