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 Stones
Wondrous item