Call the cops and talk to your roommate about the shifty friends he had over.
Yep. Ask your roommate who he had over and make him find out who took it and get it back.
That's your best bet. Your roommate owes it to you to find out who took it.
You should be able to have your personal belongings in your own home without worrying about your roommate having disgusting people over that are going to take your expensive possessions.