ch 30

He'd sustained that Transfiguration for seventeen days, and would now need to start over.

Could've been worse. He could've done this fourteen days later, after Professor McGonagall had approved him to Transfigure his father's rock. That was one very good lesson to learn the easy way.

Note to self: Always remove ring from finger before completely exhausting magic.

This implies to me that that his finger would've gotten wrecked if that was a rock. Remember, finite on the rock blew out both the front and back of the troll's head. It wasn't just expanding in a confined spot; it expanded fast enough to blow out both sides. Explosive de-transfiguration. Certainly enough to tear off a 10 year old's finger.