The Moon Crystal is a purple crystal that holds various supernatural powers. Its primary power is that it can upgrade Luna Girl's Luna Magnet into a Luna Wand, giving her a significant power boost in the process.
It was originally located on the Moon. In "Moonstruck: Race to the Moon", Luna Girl used the powers of the Harvest Moon to finally go to the moon and find the crystal. She used its powers to construct herself a Lunar Fortress. The PJ Masks were eventually able to take the crystal from her. Afterwards, they kept it locked away in the vault of their HQ, but on several occasions it has been stolen by Luna Girl or other villains to cause havoc. The last time this happened was in "Moon Madness". At the end of this episode, the crystal was seemingly crushed by the remains of Luna Girl's mega magnet, and the PJ Masks believed it to be gone for good. Mothzuki, however, noticed the crystal was, in fact, still in one piece, so it will likely make another appearance in the future.
In "Moonbreaker", it was revealed there was a second Moon Crystal. Romeo tried to obtain it, and threatened to break the moon in half while digging for it. This second crystal was lost when Luna Girl and PJ Masks teamed up to stop Romeo.
Then in "Luna and the Wolfies", it was revealed there was a third Moon Crystal. It was owned by Luna Girl as her back-up source of power if somebody succeeded to steal Luna Girl's Luna Magnet. It is unknown where and how Luna Girl obtained the third Moon crystal.