PURPOSE: To manufacture outer ornament parts made of hard gold alloy practically free from scratches and capable of permanently maintaining the beautiful brightness of gold alloy by nitriding Cr to harden the surface of a gold alloy having a specific composition consisting of Cr, Cu, Zn, and Au.

CONSTITUTION: A gold alloy having a composition consisting of, by weight, 1-15% Cr, 3-20% Cu, 1-7% Zn, and the balance Au is subjected to nitriding treatment by a gas nitriding method, by which Cr in the surface layer of the gold alloy is nitrided. By the above procedure, the surface hardness can be regulated to about Hv700. By this method, the outer ornament parts made of hard gold alloy hardly scratched by usual handling and carrying and capable of permanently maintaining the beautiful brightness of gold alloy can be obtained.