Sending stones come in pairs, with each smooth stone carved to match the other so the pairing is easily recognized. While you touch 1 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.
Each sending stone contains 50 charges. Each dawn they divide the number of charges equally, then both restore 1d6 charges. If 1 of the stones in a pair is destroyed, the other 1 becomes nonmagical.
Sending: You send a short message of 25 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 INT 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% chance that the message doesn’t arrive.