These amulets were created by Luminaire Foscari and Dr. Collodi, presumably from the world tree sapling; by resonating with the innate mana field of the person wearing them, they create a single-person-sized shield against the miasma that allows individuals to venture into the miasma without fear.
Cynthia now knows how to use a ritual to make these out of other materials, such as coral. They require a cutting or bark scraping from a living world tree, as well as organic material to channel and radiate the magic from that cutting or scraping into a personal shield.