Mechanical properties of formation rock from a subsurface reservoir are measured with a computerized system while a core sample from the formation is being cut, during a process known as slabbing, for other analytical purposes. Forces exerted during cutting of the slab from the original core sample are sensed and stored in the computer system. The recorded force data, cutting time and dimensions of the core sample and the cut slab are processed in the computer system. Measures of characteristics and mechanical properties of the rock, such as rock strength and angle of internal friction, are obtained with the computer system. Separate and specialized testing procedures performed on test core plugs using samples specially extracted from the original core sample are not required.