The platinum alloy of choice for a long time in the U.S. was Platinum
900/100 Iridium, an alloy containing 900 ppt platinum and 100 ppt
Iridium. This alloy was stamped IRIDPLAT. When the platinum stamping
regulations were revised, this alloy was then stamped 900Pt or
900PLAT or Pt 900. There is still a large amount of jewelry being
produced using this universal alloy.