The Crystals are contained inside the walls of two caves where Moon Pools are located. They are located in Mako Island and Sea Caves of Ireland. They hold moonlight, and when the Comet Eva is near the Earth, the stones create a Water Tentacle to stop it.
In Season 3, Bella Hartley moved to the coast when Emma Gilbert 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. After Emma left, 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.
- While very similar in look and function, the Crystal Necklace stones is different than the Moon Ring stones.
- As of Mako: Island of Secrets, Rikki no longer wears her crystal necklace, possibly due to her disconnecting with both Cleo and Bella.