The invention is directed to a material and a method of producing the material that is unaffected by the low-temperature degradation, humidity-enhanced phase transformation typical of yttria-stabilized zirconia in general, as well as of yttria-stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP). Because of the high fracture toughness and high mechanical strength, this class of materials is widely used, including as implants, such as for the packaging material for small implantable neural-muscular sensors and stimulators. The destructive phase transformation is eliminated by converting the surface to stable cubic or T-prime zirconia by post-densification thermal treatment in a cation-rich milieu.