Karyn was halfway to Biff's house when suddenly she stopped. Where was she going again? And why did it feel like she needed to get there as soon as possible? Shaking her head, she turned towards a different direction and headed home.
Thanks to Jon's revised history, where he was now a cheerleader, Karyn was no longer close friends with him (she usually tried to stay away from the cheerleaders). And thus, she no longer remembered the stone, so she had no impulse to get it back.