A Moonlight Amulet is a piece of jewelry with a black kyanite gemstone enchanted by witches to allow werewolves to control their transformation, thus sparing them the need to turn every month on the full moon. Their vampire counterparts are the Daylight Amulets, which prevent vampires from burning in sunlight.
It was first believed that in order for moonlight amulets to function properly, they have to be enchanted by a witch on the night of a full moon. However, it was later shown that the witches under Esther's control did not need the full moon for their amulets to be effective.
The first batch of moonlight amulets made in the modern day was crafted by Genevieve, who led Niklaus Mikaelson to believe that she was making them for Arielle Labonair, Hayley Marshall, and the rest of the Crescent werewolves. However, it was revealed that she had actually double-crossed Klaus and tricked him into allowing her to use his blood to bind the spell, since he can control his own transformation as a hybrid; because of this, the wearer of the amulets is given the power by drawing on Klaus' own strength, weakening him whenever a werewolf calls on the stone to prevent their transformation on a full moon. Genevieve also betrayed Klaus a second time by giving the stones to the newly-triggered Guerrera Pack, led by Francesca Guerrera. After killing all the werewolves who possessed the rings Genevieve created, Klaus' strength was no longer drained from those who wore them. The stones were then completely destroyed by Elijah, who used the elements of fire and water to dissolve the enchantment for good.
After her resurrection by possessing the Harvest girl Cassie's body, Esther began supervising the mass production of her own brand of the rings by the French Quarter Coven witches under her control, as she originally invented the spell for the stones, and did not need Klaus's blood for her rings to work. Also, unlike their predecessors, the stones were placed on silver rings, which is ironic because werewolves in legend grow weak from silver. However, in reality, werewolves heal from wounds caused by silver, and the choice of setting was likely based on style rather than function, as Elijah Mikaelson joked that the original batch of moonlight amulets, which were set in gold rings, were "gauche" like the Guerrera werewolves who wore them.