The diamond play button would sooner be made out of cardboard spray-painted blue than out of real diamonds. Real diamond is pretty expensive. Most people can't afford a diamond wedding ring, let alone the giant hunk of diamond it would take to make a play button. YouTube's not gonna splurge that much money for its creators. lol
Not a stupid question, though. I've read that it's actually made out of a silver-plated metal with a crystal on it to make it look diamond. And no, there's no financial reward. The only financial reward YouTube gives out comes in the p**s yellow circle we call Demonetization.