"You say that but I'm from somewhere that magic doesn't exist, so I'm liable to accidentally explode it somehow. Or it won't respond to me. That's... that's actually what I'm afraid of. That it won't recognize me as a power source." Maybe he has to be pure of heart or something to make it work. Carlisle was a healer, he was a good person. Pratt is not. And the thought that he might not be able to get the stone working again because the magic somehow knew he was a terrible human being frightened him. He'd rather not try and therefore not know.

But instead they're apparently doing science experiments with goop.

"So what's supposed to happen here?"