This paper presents an experimental examination of the initiation of tensile fracture under a two-dimensional normal wedge indentor. In addition, the nondestructive technique of electronic speckle pattern interferometry (ESPI) monitored the failure process in medium strength (Berea sandstone, 50 MPa) and high strength (Sioux quartzite, 400 MPa) rock. The results show a good agreement between the cavity expansion model and the experiments in terms of indentation pressure and size of the damage zone located beneath the indentor. A localization of microcracking was identified from the high-resolution image of ESPI. It appeared that an intrinsic length, which may be interpreted as a material parameter, developed during indentation. This intrinsic flaw or defect length controlled tensile crack initiation at the interface between the damaged and intact rock. For successful fragmentation, the initiation of tensile cracks must eventually lead to progressive chipping. Therefore, the intrinsic length could be used as an index property to evaluate the cuttability of rock.