The influence of mineralogy and moisture content on mechanical behaviour of a claystone rock is studied by the way of uniaxial and triaxial compression tests and microscopic observations. Some parameters characteristic of phenomena like plasticity and induced anisotropic damage are discussed as a function of these two factors. Rock behaviour becomes more brittle when calcite content grows or when clay or moisture content decreases. At the microlevel, plasticity is induced by slip of clay sheets and induced anisotropic damage appears by growth of oriented microcracks at the interface between grains and matrix.