Faulty finishing?

I made and sent an order of two hammered finish rings in sterling silver to a customer about 6 weeks ago. One ring 2mm wide one 3.5mm wide.

They left me a nice review so the rings must have arrived in a good condition. I use luxi white polishing compound for my final polish and am usually very careful to clean off all the polish before packaging but sometimes a slight residue can come off on the finger but not to the extent in the picture.

Today the customer sent me pictures of the ring with black discoloration around the inside edge. They said they had only worn the ring for a few days as have just got married. No problem with the wider ring. I replied asking if it has come in contact with any chemicals, perfumes etc. They said no and believe the finishing of the ring to be faulty.
I have offered to send them a free silver polishing cloth. They wish to return the ring so I can make a replacement, I would not make this any differently.

Any ideas what might have caused this?

(Sorry for the poor quality photos.)

I did melt down and recast the silver for both these rings and am now doubting myself that this might be the cause?

Any thoughts much appreciated!

There are 101 reasons this might have happened and some the customer will not admit to.
I would offer to repolish it and pay postage, with the proviso, that should it recur you will make it again with new silver. Dennis.

Thank you Dennis, I will do that.

Hi EllenAli, did you have any luck with this? one of my clients just had this happen and I cant think what it could be appart from a reaction to the skin or a product of some kind, it polished off no problem, its just really strange, I have never experience this before either.. I was thinking possibly fire stain but I looked under diffrent lighting and there is nothing.

If it left you in perfect condition then it will be either because of the oils in the customers skin or some product they have exposed it to - perfumes,oils etc.

There’s nothing that a maker has done that will make a customers finger go black after they receive it. I do wish they’d be honest. Even a residue of polish would have washed off in the course of a day in the unlikely event that had been the cause