  We report on the high-pressure synthesis of novel nano- and microcrystalline
high-quality diamonds with luminescent Ge-related centers. Observation of the
four-line fine structure in luminescence at 2 eV (602 nm) at temperatures below
80 K manifests a high quality of diamonds. We demonstrate germanium and carbon
isotope shifts in the fine structure of luminescence at 602 nm and its
vibrational sideband which allows us to unambiguously associate the center with
the germanium impurity entering into the diamond lattice. We show that there
are two ground-state energy levels with the separation of 0.7 meV and two
excited-state levels separated by 4.6 meV in the electronic structure of the
center and suggest a split-vacancy structure of this center.
