Communication Pendants also known as Communications Stones are communication devices used by the librarians.
The communication pendants are technology in the library that Jenkins was trying to make operational. Jenkins used copper to attempt to complete the pendants which Flynn described as an interesting choice. There is a microphone transmitter in the annex that can be used to communicate with the pendant. The librarians used the communication pendants on their mission to recover the Eye of Zarathustra. When Flynn Carsen was captured by Ray Glasney he used is the pendant to communicate and send a secret message in Chinese telegraph code to the other library. Ray destroyed Flynn’s communication pendant.