Poseidon's Pearls

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

The Lightning Thief
In The Lightning Thief, as Percy jumps into the Mississippi River, a Nereid tells Percy to go to Santa Monica. When he arrives, the Nereid gives him the pearls saying they are gifts from his father (Poseidon). Percy takes the pearls to the Underworld. And Hades tries to kill them 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 Nereid said.

The Lightning Thief (film)
In the movie, the pearls were said to be Persephone's instead of Poseidon's, and were used to help her visitors escape the Underworld. Percy, Annabeth, and Grover had to travel all over the U.S to retrieve these pearls to rescue Percy's mom. The first was located in Aunty Em's Garden Gnome Emporium. The second was located in the replica Partenon in Nashville, TN. The third was located in the Lotus Casino in Las Vegas.

Effect/Powers
The pearls' main power is to return the user back to the sea. When the pearls are smashed by the user, an impenetrable bubble encases them. The bubble is able to float up and go through solid rock (as demonstrated when Percy used his). 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 them; this replaced Percy and his friends flying home on a plane since the pearls brought them to Manhattan instead.