a long time ago I read a book about a dragon that took a young girl through space to a planet that she was from. I don't remember much but I do remember that there were dragons who were killed for a powerfull crystal that was grown in their skulls. There were also dragon like creatures who's crystals were bad. The girl had friends who tried to get her to remember her family and they took her to a palace were a door was opened and a mural disintegrated because it was so old and the change in air pressure disturbed it.
I really don't remember much more but I keep thinking about this book and have been looking for years. Please help me.