Rubber isolation systems represent an effective method to isolate the superstructure from seismic ground accelerations. In this study, two sets of high-damping natural rubber bearings are subjected to characterization tests to determine the corresponding stiffness parameters, Bearings with flat steel laminae represent the first set, while the second one has inclined steel plates necessary to obtain a transversely nonisotropic response. During shear deformation a negative hydrostatic stress state and irreversible material damage in the form of internal rupture develops. The cavitation region is not visible from the outside and thus not recognized or accounted for during experimentation. Numerical simulation using a cavitation damage model based on a variable bulk modulus quantifies the amount of stiffness reduction attributed to internal rupture in the material. It is shown that the amount and extension of damage is much more severe for elastomeric bearings when inclined steel plates are used to generate anisotropy in the in-plane response.