Does the letters DIA that form part of the hallmark on a gold diamond ring mean it has an authentic diamond.:)

I don't know about the DIA but a real diamond will scratch a mirror. Diamonds are one of the hardest and most resilient materials in nature; a diamond can only be scratched by another diamond. So try it out on a mirror to make sure, or take it into a jeweller to look at. None of my daimond rings have this designation, except one has a tiny polar bear cut into it to signify it is a Canadian diamond.

Normally if the diamond is authentic , where the DIA is and the other hallmarks, there should be some numbers like 17 or 25 e.g. no matter how big or small the diamond is, it will have some numbers. I know some of our pieces have had dia on the inside of rings and that's for diamond, as well as the numbers saying the weight, but sometimes it can be the manufacturers mark too. Best to take it into a reputable jeweller and get them to look at the marks.