Under a wide range of conditions, we directly measure the stiffness in the shear loading direction of five direct-shear apparatuses employed in the Marine Geotechnics laboratory at MARUM, University of Bremen. For the stiffness measurements, a steel blank simulating a rock sample with dimensions of 25 mm height and 25 mm diameter was placed in the sample cell and loaded in the same manner as geologic samples. A vertical load was applied to the top face of the steel cylinder corresponding to normal stresses (σn) 1, 4, 7, 10 or 14 MPa. For G5, additional normal stresses of 20, 30, and 48 MPa were tested. At each normal stress, stiffness was measured for both a loading and unloading phase. For each normal stress, the range of shear stresses tested was designed to nearly match the range of the applied normal stress; for example, for G2 at 10 MPa normal stress (Figure 4), the sample was loaded over a shear stress range of 1-9 MPa. We designed the shear loads in our measurements to cover a range of equivalent friction coefficients, so that an exact apparatus stiffness value could be matched to any particular geologic sample.