Bob's collar is a collar from Bob the cat. It is used in Ritual of the Mahjarrat. One side shows Bob, other side DIRAKS, which is the Fairy ring coordinates to get to Kethsi. In Kethsi you have to use the collar on the device. On the device to match up the pattern you have to set Bob side vertically between the third and fourth patterns.
The collar can be returned to Bob after the quest. Find Bob (if necessary, use the Catspeak amulet (e)), talk to him about Ritual of the Mahjarrat, and choose the collar option. The collar can not be reclaimed from him after returning it.