The stone morphed into a girl. Or rather, a life-size statue of a girl. Instead of creating a new girl out of nowhere and changing countless little things to accomodate her into existing history, it was easier to become an inanimate object, complete with a retroactive backstory about how Jon's grandfather found "her."
Jon was shocked. Not only was this not what he had in mind, but he also couldn't get over the fact that "her" face seemed so familiar. Why? After a few seconds of staring, he figured out the answer: Her features - in fact, her whole form - was an amalgam of Zoe, Karyn, and even Sarah McMillan, the three girls that dominated his thoughts. The stone preferred not to use "its" imagination whenever possible, drawing upon its user's mind to "fill in the blanks." So an unspecified "girl" would be like the girls he thought about the most.
