Panic Switches are one of Dex's many inventions.
When someone presses the gem in the middle of the ring, it alerts Dex that that person is in danger. The first iteration allowed the wearer to signal that they were in danger by pressing the stone in the center. Dex later added the functionality of being able to track where the wearer was without the wearer needing to press the stone.
Dex has mentioned that he is reworking them in order to allow the wearer to specify which type of danger they are in.
In Everblaze, Dex gave Sophie a panic switch after class in the locker room, stating that he had been working on it for "a while." Dex made Sophie promise to wear it all the time, saying that he wanted to be sure that she was safe. It was incredibly awkward when Dex gave Sophie the panic switch, as everyone was looking at them.
In Lodestar, Dex gave panic switches to Biana, Fitz, Tam, and Linh. These panic switches contained stronger trackers, coupled with the ability to track the wearer's location without the wearer needing to push the stone. Dex also stated that he was working on a more specific system — possibly adding the ability to push the stone in four different directions to signify different types of danger.
Sophie's panic switch is described as a plain silver band with a crooked, flat beige stone set into the middle.
Dex's has a matching silver band.
Biana's panic switch is said to have a sparkly pink stone on the band.
Fitz's panic switch is said to have a teal stone on the band.