Jon thought about it, then decided to take the necklace. Athena could have been right. While Jon figured that Athena might disappear after taking off the necklace (Zoe disappeared, so it made sense to think that Zoe's friend would disappear too), he couldn't take the chance to lose the stone forever. Sure, they needed Athena's help, but if the stone disappeared, what was the point of all this?
"Okay, I'll take it," Jon said.
Athena nodded and took off the pendant necklace.
