Signal Rings
Wondrous Item,
Common
(cost 60 gp)
Crafting Components
Pair of ship’s signal flags that have been used for at least a year and a day
These rings come as a set of matched golden bands with a green enamel inlay. As an action, the wearer of one can rotate the inlay a full turn, at which point it turns to red. When one ring changes color, the other one does as well, regardless of its location.
Source