Surface nitriding of tetragonal zirconia (t-ZrO2) has been performed with the aim of combining the optical properties of zirconium nitride (ZrN) with the hardness and toughening properties of tetragonal zirconia. A high current d.c. arc plasma source has been used to create a reactive environment of atomic hydrogen and NH radicals. Plasma analysis showed that a highly reductive atmosphere is necessary to transform the tetragonal zirconia. X-ray diffraction and transmission electron microscopy analysis show an efficient transformation of the t-ZrO2 at the surface which exhibits the typical yellow-gold color of ZrN and high wear resistance. For a 2 h treatment at 750 ℃ the nitriding is accompanied by the formation of a 250 nm thick gradient layer of ZrN structure and a partial transformation of t-ZrO2 into cubic zirconia deeper in the bulk.