Yep, that's a Tapatalk specific icon - nothing to do with the Friendly forum. According to a thread in Tapatalk's support forum, it means you have "VIP" status, usually from purchasing a subscription of some kind:
Diamond Icon by UserID?
If you haven't intentionally subscribed to some sort of Tapatalk service within the last year, you might want to look into it a bit more. It's possible you may have accidently purchased some sort of feature with "automatic renewal".