In the manuscript we report on characterization of single-crystalline (111) plates prepared from type Ib diamonds with nitrogen content of 100-150 ppm and (100) plates prepared from IIa diamond by means of high-resolution rocking curve imaging (RCI). Contrary to a common opinion about intrinsic poor diffraction quality of type Ib diamonds, RCI showed the presence of nearly defect-free areas of several mm2 in the central part of the (111)-oriented diamond plates. In comparison with the (100)-oriented IIa diamond plates prepared by the same HPHT setup the Ib diamonds possess better diffraction quality. The observed broadening of the rocking curves associates with the cutting and polishing processes, causing strains around the edges of the plates and rare defects. An improvement of preparation technique will thus allow to make single-crystalline diamond plates for Laue and Bragg monochromators and beam splitters from type Ib diamond with areas large enough to be used as wavefront-preserving optical elements at 4th generation synchrotron facilities.