I remember reading a book early 2000s, it was a fantasy novel from my high schools library. It was about a girl transforming into crystal that caused her excruciating pain. She was traveling to a crystal castle/pyramid/palace on a boat and during the trip one of her companions could help her by taking her pain and experiencing it himself for short periods. I believe that at the end she integrated into the castle itself. I also have Neil gaiman in mind when I remember it but I don’t see it in his book list.