I did receive a light grey-coloured Diamond membership card in the mail. I think I might have received a small passport holder, but my memory is a bit fuzzy. That could have been before Diamond was introduced. If there was anything else sent along with that, it certainly wasn't very memorable.

The only tangible benefit of Diamond, was the 100 Euros in dining certificates that were never provided. Otherwise, it's the same as Platinum. I suppose if you want to really get picky, another advantage of Diamond is that if you fail to requalify, you will soft-land to Platinum, which gives basically the same benefits as Diamond. So, consider reaching Diamond to equal 2 years of Platinum without needing to requalify.