A tensorial measure, called the crack tensor, is proposed to objectively define the crack geometry in crystalline rock as general as possible. Change of crack populations during brittle failure of rock (Inada granite) is analyzed in terms of the crack tensor and its invariants, which can be evaluated by a conventional petrographic analysis combined with stereology. A crack evolution law is formulated as a function of inelastic strain, so that we can easily depict how much damage accumulates in a sample loaded in a triaxial vessel by measuring the inelastic strain. When rock is loaded up to failure, the crack population satisfies a unique relation in the plot of the crack density versus the anisotropy, which are defined using the invariants of the crack tensors. It is suggested that failure of rock in uniaxial compression may be determined by a local condition such as a stress intensity factor at a crack tip. In the triaxial tests, on the other hand, rock fails when the crack density increases over a threshold value, which is a global condition depending on all cracks existing at failure. The threshold density is about 7 for the rock subjected to differential stress under confining pressures larger than 25 MPa. Under such a high crack density, the rock does not behave like a cracked solid, but rather like an assembly of rock blocks (granular material), and it starts to fail mainly due to the structurally induced instability, not due to the critical stress intensity factor at a crack tip. A throughgoing fault may develop as a result of the relative motion among the disintegrated blocks, sliding and rolling.