Given to him by the demon Graulas, the seeing stone allows the cliff troll Nero to see most areas of the Fablehaven preserve. By definition, it was patterned after the Oculus, but not nearly as powerful.
Seth, Kendra, and Ruth need to find Grandpa and Lena, so they go to Nero to use his seeing stone for help. Nero agrees to help in exchange for a massage from Grandma. He later tells them that Lena and Grandpa are in the Forgoten Chapel.
Grandma, Grandpa, Dale, and Hugo go to Nero to find out more about the figure that was released from the Quiet Box, but find Nero collapsed under a log, with his seeing stone gone.