A seed crystal for the fabrication of a superconductor is grown from a rare-earth oxide having the basic formula XwZtBaxCuyOz, X comprising at least one rare-earth element and Z being a dopant which raises the peritectic decomposition temperature (Tp) of the oxide. In a preferred embodiment, the dopant is Mg. Use of this rare-earth oxide material for seed crystals increases the temperature at which cold-seeding can be performed and thus enables the growth of a wider range of bulk superconductor materials by this process.