I was about to post this heh. Also I have some additional information. When you trade them it says "You have the maximum number of those gems in your inventory or socketed into items." even though it is not a gem. The mugs also are unique, but it allows you get have more than one, which is odd.
http://img268.imageshack.us/i/tradem.png <- Shows it is unique yet I have more than one
http://img268.imageshack.us/i/socket.png <- Shows the error message it gives when you try to trade it