All Travelers and Acolytes have grey rings that are seemingly carved out of stone. These rings have various symbols around their edges which represent each of the territories.
A Traveler's ring will become crystalline and glow when its wearer is close to a Flume. Made of the same material as the Flume, two rings can link to make a miniature Flume of sorts between them. When activated to send or receive, the rings will react as they do when near a Flume but with the addition of tinkling music. Bobby uses this gateway early on in his adventure to send journals to Mark- and later, Courtney- his "safe keepers".
The Rings can transport small items in the small flume, like how Bobby sends his journals to Mark and Courtney. They can also be used for communication between Travelers, like in the end of Book 3, or beginning of Book 4, when Aja Killian contacts Bobby to bring him to Veelox. Acolytes can use their rings to send notes to other Acolytes, too, like how Mark sent a note to Wu Yenza, saying that Saint Dane brought the Cloral poison to Eelong.