The necklace appears to be an average necklace with four, vaguely spherical beads with spirals dotting the bottom side with a much larger, pseudo-diamond shaped spiral bead at the very tip.
After a failed game of Legball, Marcella gave Leo the necklace as a gift for being kind and sweet to her, being the only one who supported her while the rest of the citizens insulted her for being a witch. Leo graciously accepted the gift and saw her off while the two went their separate ways. During the Conquering of Puebla, the Necklace would protect Leo from the destruction of Quetzalcoatl's Portal caused by the Mystic Bomb. Although shocked that Marcella had given him a magic charm, and even more shocked that she actually was a Hechicera, Leo was none the less grateful and looked passed her magical ways to thank her for saving his life.
The necklace appears to give the user an aura of protection against magical spells, as it protected Leo from the Mystical Explosion. It does not appear to protect the wearer from more earthly assaults such as beast, mechanical weaponry or so on.