Registry Pendants are the way the Council keeps track of the locations of everyone in the Lost Cities. Each elf who lives in the Lost Cities is required to wear one so they can be tracked. In Foxfire, they are used to take attendance. There are, however, ways to mess with the registry feed through Technopathy. The cords on Sophie and Dex's pendants were made thicker after they were found at the end of Book 1: Keeper of the Lost Cities.
They are made of metal and a crystal, which holds balefire.
They are described to be a choker, with a thick silver cord. In the center, there is one pendant, a silver hoop with a small crystal in the center. It can be seen in the family portrait on Sophie. Many of these pendants vary in shape, such as how both Sophie And Dex have two cords instead of one after being kidnapped by the Neverseen in Book 1.
“He handed her and Dex new registry pendants. They were fancier than their old pendants. Small copper beads framed each side of the crystal--which was now a triangle with three facets on each side--and extra cords had been woven into the chain, making it triple thick.”