Jareth handed her the crystal, knowing that she would be able to see the woman's face inside of it. Were she able to tell him who THE WITCH was, she would be allowed to keep the crystal, if she was quite specific. Jareth, however, had a tendancy to downplay the importance of human information, so his opinion of whether the information was important enough to be given one of his all-important crystal balls or not would be quite a delicate thing.