Poseidon's Pearls

Poseidon's Pearls are three magic pearls gifted to Percy Jackson by his father Poseidon and were delivered to him by a Naiad in The Lightning Thief.

The Lightning Thief
As Percy jumps into the Mississippi River from the Gateway Arch, a Nereid, sent by Poseidon, tells Percy to go to Santa Monica. When he arrives, a Naiad gives Percy three pearls saying they are gifts from his father, Poseidon. Percy takes the pearls with him to the Underworld. Hades tries to kill Percy, Annabeth, and Grover when they smash the pearls with their feet, the pearls then carry them back to the Santa Monica beach because "What belongs to the sea returns to the sea" as the Naiad said.

The Lightning Thief
The pearls are said to be Persephone's instead of Poseidon's, and are used to help her visitors escape the Underworld. Percy, Annabeth, and Grover have to travel all over the United States to retrieve these pearls to rescue Percy's mother. The first is located in Aunty Em's Garden Gnome Emporium. The second is located in the replica Parthenon in Nashville, Tennessee. The third is located in the Lotus Casino in Las Vegas.

Effect
The pearls' main power is to return the user back to the sea. When the pearl is smashed by the user, nothing will happen for a few seconds. When Percy was in the underworld, he used a pearl, and a bubble surrounded them, bringing them up from the underworld and near a beach, but still out to sea. Hades calls them Poseidon's "little tricks".

In the film they don't return you to the sea, but to the place you visualize, when you crush the pearls; this replaced Percy and his friends flying home on a plane since the pearls brought them to Manhattan instead.