Sand Dollar

A Sand dollar is a type of shell found on beaches when they are washed ashore by ocean waves. It can magically clean water pollution as shown in The Last Olympian when Percy met both The East River spirit and The Hudson River spirit during the Battle of Manhattan to sink Titan boats. The sand dollar might have many other uses as well.

The Battle of the Labyrinth
Percy gets a sand dollar as a gift for his 15th birthday, from his father, Poseidon. Not knowing when or how to spend it, or what he was even supposed do with it, Percy adds it to his camp necklace.

The Last Olympian
Percy uses his sand dollar during the Battle of Manhattan. He gave half of it to The East River spirit and the other half to The Hudson River spirit, in exchange for keeping Kronos' armies from crossing the East and Hudson rivers respectively. Despite it's cleaning power, Percy noticed that each river's pollution proved to be too thick and constant to clean them.