The Crystals are "true" moon rocks that are inside the walls of two caves, which were created when a comet fell to earth. 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, and when the Comet is near the Earth, the stones create a Water Tentacle to stop the comet.
In Season 3, Bella Hartley replaced Emma Gilbert, who left to go travel the world with her family. In the previous seasons, the three mermaids: Cleo, Emma and Rikki wore 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 immense energy to cause power outages of a city, and a blue light that mesmerizes the girls in the same way as the Full Moon.
- These stones might be the same as the stones found in Moon Rings, as they both appear to hold moonlight and can simulate the effects of a full moon.
- This stones are extremely rare and is an item of great value.