I made a platinum diamond solitaire ring a year ago with a rub over setting. Now the customer has come back unhappy as the solder joins are turning black.

I have made other rings with this medium platinum solder from Cooksons with no problems of discolouration. I also tested the ring with acid to check I had not used white gold by mistake but the acid stayed clear showing it was platinum solder.

Does anyone have any advice on this. If I rhodium plate it I presume this will wear off and the black will come back.

Thank you Richard