  Wavefront-preserving x-ray diamond crystal optics are essential for numerous
applications in x-ray science. Perfect crystals with flat Bragg planes are a
prerequisite for wavefront preservation in Bragg diffraction. However, this
condition is difficult to realize in practice because of inevitable crystal
imperfections. Here we use x-ray rocking curve imaging to study the smallest
achievable Bragg-plane slope errors in the best presently available synthetic
diamond crystals and how they compare with those of perfect silicon crystals.
We show that the smallest specific slope errors in the best diamond crystals
(both freestanding or strain-free mounted) are about 0.15-0.2~$\mu$rad/mm$^2$.
These errors are only a factor of two larger than the 0.05-0.1~$\mu$rad/mm$^2$
specific slope errors we measure in perfect silicon crystals. High-temperature
annealing at 1450$^{\circ}$C of almost flawless diamond crystals reduces the
slope errors very close to those of silicon. Further investigations are
required to establish the wavefront-preservation properties of these crystals.
