In Everblaze, Dex gives Sophie a Panic Switch ring, that he'd been making for a while, after class in the locker room. When someone presses the gem in the middle of the ring, it alerts Dex that that person is in trouble. Dex makes Sophie promise to wear it all the time, saying that he wants to make sure she's safe. Keefe later teases her about it saying, "No wonder Dex gave you a ring. Smart and beautiful."
Later, in Lodestar, Dex gives panic switches to Biana, Fitz, Tam, and Linh with stronger trackers so he can hone in on the signal even if they don't press the stone. He says that he is working on a more specific system, possibly being able to push the stone in four different directions that mean different kinds of danger.
Sophie's panic switch is described as a plain silver band with a crooked, flat beige stone set into the middle. Dex has a matching silver band. Biana's panic switch is said to have a sparkly pink stone on the band.