One stone is currently held by Sabine Wade, the other is held by Tylanys Dawnstar.
Wondrous item, uncommon
Sending stones come in pairs, with each smooth stone carved to match the other so the 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 know that fact as soon as you use the stone and don't cast the spell.
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 nonmagical.
Sending
You send a short message of twenty-five words or less to a creature with which you are familiar. The creature hears the message in its mind, recognizes you as the sender if it knows you, and can answer in a like manner immediately. The spell enables creatures with Intelligence scores of at least 1 to understand the meaning of your message.
You can send the message across any distance and even to other planes of existence, but if the target is on a different plane than you, there is a 5 percent chance that the message doesn't arrive.