Planar impact methods and velocity interferometry diagnostics have been used to measure the spall strength of Solenhofen limestone and Dresser basalt following shock precompression to stresses over the range of approximately 100 to 700 MPa but below the Hugoniot elastic limit of both rock materials. Spall strength and spall fracture energy are determined from the measured spall pullback profiles. Spall strengths of 77 MPa for Solenhofen limestone and 130 MPa for Dresser basalt, independent of impact amplitude, were determined. ©2000 American Institute of Physics.