Mixtures of alkaline earth carbonates together with nickel or copper oxide and iridium, ruthenium. platinum or rhodium metal in a carbonate Bur at 1050-1150 degrees C leads to the formation of black crystals. These crystals, whose habit is either hexagonal plates or hexagonal rods, belong to a family of perovskite: related phases of general composition [A](x)[(A',B)O-3]. Preliminary diffraction experiments indicate that these crystals form with a composite structure, some commensurate and others incommensurate. The structures of the commensurate crystals were determined by single-crystal X-ray diffraction, while the structures of the incommensurate crystals are being investigated by powder diffraction. Several commensurate and incommensurate single crystals and their growth conditions are discussed in this paper.