Reason Why One Diamond in Halo Ring is Not Shiny

Okay, so I got my ring in January and it was spotless save for a scuff on the back of the ring (that mysteriously is no longer there). If it had a mark on any of the diamonds I would have noticed. I also have pictures showing it was fine. Sometime the week of or the time after my wedding, one diamond got very very dull. I went to get it cleaned and when they checked under a microscope they could see it was very dull, but they were not sure if it was the diamond itself or just something on it.

They cleaned and scrubbed it, as have I, and nothing makes it any brighter. I know the ring wasn’t like this when I got it, as it only shows up in recent pictures.

Has any one else had this happen? I got my ring from Zales if that helps. Their only solution was to try to just give me the twin of my ring in their store- but I’m sentimental so I’d like to keep the one I got married in. I even wonder if before I got married it some how got switched when I brought it in to get cleaned, since that scuff on the setting isn’t visible.

Or maybe I’m crazy? All I know is I got married in this one (even if it wasn’t my one I was proposed to with) and I’d like to keep it, and not risk them possibly switching it if I send it to get fixed.