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.