This invention describes a crystalline alumina loaded cavity resonator which has low loss and high frequency stability such that its frequency is well-defined and only weakly perturbed by temperature, pressure and mechanical changes in its environment. Basically the resonator is a single crystal of sapphire (1) having protusions (2) and (2') fitting closely into recesses in the base (3) and lid (3') of a niobium housing. The lid (3') is clamped by groove (4) and having a indium seal to seal the lid (3') to the side walls (5) at groove (6) in lid (3'). A microwave probe (7) is used to couple microwave power into the cavity through hole (8).