The Stone Prince was a prince who was sent to Hero's School, and everyone else went home and started achieving great feats, whereas he didn't. Even the kids no one thought would do anything, like Jack, who just wanted to work on his mom's farm and ended up killing a giant and becoming rich.
So, he did decided to do services for Kings, and then everyone was asking him to do services, when he only needed to do one great deed, they were annoying him all the time, so he just went to the caves of fire and ice to cure a king with the waters, and there were two ladels, a old, ugly tin one and a beautiful gold one encrusted in jewels.
He knew he was supposed to choose the tin one, but he thought it wouldn't hurt if he just held the gold one, since it was so nice. However, he turned to stone when he started touching it, and then he plunged himself in the waters, and so, he became the stone prince. He helped defeat the wizards Zemenar and Antorell, and married Alianora.