Immediately after Jon became the Goth girl Jen, Zoe witnessed a weird purple bolt of electricity form between the heads of Karyn and Jen. They froze for just a moment, until the electricity faded away.
"What was that?" Zoe asked.
Jen didn't know what her sister was talking about. And neither did Karyn. Jen stood up from the floor, still holding the stone in her pale hand.
"You mean you didn't see it?" Zoe asked. They shook their heads "no".
What Zoe didn't know was that the purple electricity changed Jen's and Karyn's memories so that Karyn remembered Jon as Jen the Goth girl and that Jen remembered Karyn as Karyn the Goth girl. It was the device's way of keeping everything "in sync". Well, only people affected by the device, not anyone or anything else. And apparently the purple electricity only occured when people affected by the device were in close proximity to each other.
Jen looked around her room. "Ugh. What happened to my room?" It was still Jon's bedroom. The device wasn't the stone, it only changed appearances and memories of people. It didn't change reality, like the stone did.