I guess Pd and Pt are unsuitable for jewelry. Platinum is very suitable for jewelry, however for platinum jewlery to be marketed and stamped Platinum or Pt or Plat it must be 95% pure. That carries quite a price tag, especially when it holds one of those 5 carat rocks in the middle
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So I think it is a practicality issue, I am not sure of the use of palladium in jewelry, but I know platinum, rhodium, and irridium, are sometimes conglomerated into a type of platinum. I think I have seen it stamped as (ir)plat or something.