Loki's scepter couldve looked like ANYthing. But it looked like the one thing already confirmed in the universe that controls minds. The fact that it actually has something to do with Thanos only further supports the theory. He put it in Loki's hand. Is it harder to believe that Thanos would temporarily "give it up" than it is to believe there are two blue gems that do exactly the same thing in the same Universe?