A scratch tester has at least one cutter that moves simultaneously both rotationally and axially relative to the rock it is cutting. When rotational and axial movements are constant, the cutter generates a helical groove in the rock. In borehole embodiments, the scratch tester is fixed at a desired location using centralizers, and the cutter is provided on a motorized platform/track that translates between the centralizers and rotates around a central axis. The cutter faces outward and extends via a cutter arm to engage and carve a helical groove in the borehole wall. A laboratory scratch tester includes a holder for a solid cylindrical core sample and a motorized translating frame on which a cutter extends. The cutter is directed toward the core sample, and the holder with the core sample is rotated by a motor so that as the cutter translates relative thereto, a helical groove is cut thereinto.