You mean this stone?

Karyn thought faster than she ever had in her life. Sarah hadn't said which stone she wanted Jon to give her. Even before the next wish was finished, she had come up with a plan. She couldn't do any serious magic without being ordered to by her mistress, but this was fairly minor stuff. Somewhere else... "I wish I had a precious stone I could give to Sarah." Jon looked at the ruby that suddenly appeared in his hand. "Why on earth did I say that?" he thought. After a few more thoughts he said aloud: "I wish I knew what made me make the previous wish." On his way to Sarah me made some important plans and even more important wishes. One of them was obvious: "I wish Sarah wlll firmly believe that the ruby is the magic wishing stone and that nothing and nobody can ever convince her it isn't."