The invention concerns a diamond bearing assembly for a downhole motor. The assembly (10) includes at least one set of opposing, relatively rotating thrust bearing rings (20, 22) each of which includes an annular support element (24, 30) and PCD compacts (26, 32) carried by the support elements. The PCD compacts of the bearing rings oppose one another at a thrust bearing interface (80). Shear limiting means (96, 110) are provided. These means are arranged to shear if frictional forces between the opposing PCD compacts at the thrust bearing interface exceed a critical level. This eliminates relative rotation between the bearing rings and avoids possible overheating and thermal degradation of the PCD. The shear limiting means can also serve as an alignment means to ensure correct axial alignment of the bearing components. The bearing assembly may also include a compact radial bearing capability.