This invention relates to a method for forming, and constructs of, bonded composites of single crystal materials. The method comprises the steps of optically contacting the surfaces of the single crystal materials selected to form an assembly, bonding the surfaces by gradually heating the assembly to a temperature and for a duration sufficient to diffusion bond the surfaces, and cooling the bonded assembly at a rate which avoids thermal shock and allows the removal of stress by annealing. No bonding agents are necessary. Moreover, since the bonding temperature does not have to reach the substances' melting or fusing temperature, minimal distortion and displacement of the surfaces results.