Two Sending Stones with two different faces (attributes below) were found in the belongings of Daggard Stabbyns; their counterparts could be out there in the world somewhere, or even in the beyond. Daggard did not know enough to actually attempt to contact the persons that may hold the other Stones.
Sending Stones come in pairs, with each stone carved to match the other so that pairing is easily recognized. While you touch one stone, you can use an action to cast the Sending spell from it. The target is the bearer of the other stone. If no creature bears the other stone, you will know this as soon as you use the stone (as no spell will be cast).
Once Sending is cast through the stones, they can't be used again until the next dawn. If one of the stones in a pair is destroyed, the other one becomes nonfunctional and nonmagical.