Moon Crystals are purple crystals that holds various supernatural powers. They can only be found in parts of the Moon.
The Moon Crystals are able to come in different variants:
Harvest Moon Crystals
The first discovered harvest Moon Crystal 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. Motsuki, however, noticed the crystal was, in fact, still in one piece.
The crystal was eventually retrieved presumably during the events of "Moon Rocked" and "Play Date on the Moon" between the events of Season 3 and before the events of "Heroes of the Sky" by the villainess; however, the PJ Masks made no attempt on confiscating it from the villain afterwards (except for "Space Race" where it was presumably confiscated from Luna for a short time).
In "Moonbreaker", it was revealed there was a second Harvest 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. However, the PJ Masks never got their chance to get the crystal from her at the end.
Back-Up Moon Crystals are identical to Moon Crystals, except it doesn't change Luna Girl's Luna Magnet to a Luna Wand, but it is able to carry heavier objects such as the Museum.
Dark Moon Crystals
In "Moonwolfy", it was revealed that the Moon contains another variant of Moon Crystal known as the Dark Moon Crystals or Black Crystals. It was used by Luna Girl and Motsuki to turn Kevin full-wolfy that would help them to find more dark moon crystals, but the crystals were eventually confiscated by the PJ Masks.
In "Moth on the Moon", it was revealed during at a Harvest Moon, moths such as Motsuki are able to form moon crystals in caves on the Moon that allows them to house bad energy relating to arguments related to the moon in order to transform into a moth girl.
In "Heroes of the Sky", it is revealed that there are other moon crystals that allows to keep the Lunar Fortress at bay.
In "Commander Meow", it is revealed that there is a crystal that controls the Lunar Fortress' powers based on the person's energy it picks up.