Crystal Necklace

The Crystal Necklace was first worn by Bella in the show h20 Just Add Water.Then later Cleo found a crystal hidding in the rock found in the moon pool.The crystals are "True" moon rock that are inside the two cave.The two caves are now known as Mako Island and Sea Caves of Ireland. The crystals are the source of the Moon Pool's magic. They hold the moonlight.

History
In Season 3, Bella replaced Emma, who left to go travel the world with her parents. In the previous seasons, the three mermaids: Emma, Cleo and Rikkiwore lockets to help them remember how special they were, and presumably Emma continued to wear hers. However, the new trio replaced the lockets with jewel pendants, with stones found inside the rocks of the walls of the Moon Pool. When in contact with each other, these crystals generate enough energy to cause power outages, and a blue light that mesmerizes the girls in the same way as the Full Moon.