Episode title

The Moon Ring
Moon Rings store moonlight and only works in a place that can receive full moonlight, such Lyla used it to open the Trident Cave, but it did not work inside. When the mermaid graduates in the Mermaid School, she is given a Moon Ring and is initiated in their respective pod. If a mermaid loses her Moon Ring, it will be near impossible to receive a new one unless they can prove to their respective mermaid council their worth.

On the show you can not just recive the ring you must earn it (Of course its just fiction) In reality you can find moon crystals in the Sea Caves Of Ireland (Click Ireland to learn more about the sea caves of ireland)

Appearances
The moon ring was first shown on season 1 Episode 1 "Outcast" Where the Mermaids Nikie,Sirena,and Layla accidently let Zac in the moon pool on the night of the full moon