First: X-ray diffraction is a non-destructive method probing the nanometre scale, which provides various essential material parameters without the need for being referenced.
Second: the method is known and accepted for its accuracy and reliability since years in science, research, and development. In many cases just one quick X-ray diffraction measurement is required to determine sample parameters such as: layer thickness, roughness, density, and chemical composition; porosity; lattice spacing, gradients, mismatches, and degree of relaxation; preferred orientation, texture, stress, and strain – all with a local resolution down to some 10 µm square.