Mordenkainen's Improved Sending Stone

Wondrous Item, very rare
  Improved Sending Stones come as a collection of four, with each smooth stone carved to match the other so their connection is easily recognized. While you touch one stone, you can use an action to cast the sending spell from it. The targets are the bearers of other stones. If no creature bears another 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 be used again for an unlimited number of times because that’s the point of communicating - why would anyone think just once a day was going to be enough, did they think we’d just all go around with multiple pairs just so that we could talk to one other person?!?! If one of the stones in a set is destroyed, the others become non magical.
Item type
Magical
Creator