Uh, what? I didn't get the crack part...

So in order for her to get the magic to be pretty, she had to present something very important and had meaning to her and that was the stone in like chapter 4 I think. But what i think it means is that somthing bad might happen

yes, as the comment above explained, they have to give something important/precious to that person. And, to add on- I think it meant if a crack is developed, the former fate/destiny will change. So like, what was suppose to happen will not happen. I think that crystal belonged to the rich female friend.