Okay so I see lots of You are wondering, and the post below v.v too.
So I'll tell You!
The diamonds mean that the user is apart of Stardoll Royalty! Stardoll Royalty is an elite club. It is also a private club, You cannot join it, and You cannot Leave it. Stardoll must invite You. It's for people who have been a superstar for a year, possibly in a row. And when You're not a superstar anymore, Stardoll Royalty kicks You out. So yeah, it's very unfair.
You can visit the club HERE, but You won't be able to join it, unless Stardoll has invited You.