PURPOSE: To efficiently produce a precious ornament such as a ring by incorporating a specified amt. of cerium into a hard high purity platinum alloy.

CONSTITUTION: A crucible contg. 60g platinum and 0.04g cerium is put in a high-frequency electric furnace. After melting by heating for 1-1.5min, 60g platinum and 0.04g cerium are added and melted by heating for 0.5-1min, 0.04g cerium is further added and the furnace is hermetically sealed and evacuated by a vacuum pump. At the time when the internal pressure of the furnace is reduced to 0.1atm, heating is stopped and the resulting melt is centrifugally cast into a ring shape at 1,900°C casting temp. in a sand mold at 800°C formed by a lost wax process. The objective hard high purity platinum alloy contg. 10-100ppm cerium is produced. High hardness can be imparted to platinum only by incorporating such a very small amt. of a metal of a different kind.