enigmaticpenguinofdeath:

Just a quick screencap run-through of a few of the times Mycroft’s right hand is the central focus of a shot and you would have had a chance to see the ring on the fourth finger of his hand. Apparently some people fail at the first Sherlockian hurdle and neither observe nor even see, because they’re just spotting it now in series three and have jumped to the conclusion he’s secretly found himself a ‘goldfish’ and got married during the hiatus. Nope!

Mark has said before on twitter that it’s not a wedding ring, and confirmed it’s a prop and not one of the two he usually wears. There’s been meta written about it over the years; examples here and here and here. And certain other people powers that be have theorised it contains a laser. Speculate away, but it’s certainly not a new addition.