Sending stone

Sending Stones come in pairs that allow the user to cast the Sending spell to the holder of the corresponding stone.

Appearance
The stones are smooth and sometimes carved so as to be recognizable as one of a pair. The stone found in the Sutan residence was described as being a single polished palm-sized river stone. Yussa Errenis's stone was also a smooth polished river stone but light blue in color.

Properties
A creature holding one of the stones can use their action to cast Sending, targeting the creature that bears the other stone. If no creature has the other stone, the caster knows this and doesn't cast the spell.

Some sets of sending stones can cast Sending once per day, recharging at dawn. Other sets of sending stones can cast the spell multiple times, allowing their bearers to have extended conversations.

Campaign 2: The Mighty Nein
Lord Diedric Sutan had a sending stone which was stolen by Caleb Widogast during the party's break-in of the Sutan residence. The holder of the corresponding stone was Ionos Jagentoth.

Yussa Errenis and Allura Vysoren held corresponding stones. Yussa had instructed Wensforth to use it to contact help in the event of an emergency, and he did so when Yussa disappeared into the Happy Fun Ball.

Essek Thelyss gave a sending stone to Dagen Underthorn so that he could contact the party in the event that the Tombtakers arrived while the party explored the ruins of Aeor.

Campaign 3: Bells Hells
The party that would later be called Bells Hells purchased a pair of sending stones from Advik, a merchant in the Elder's Post in Jrusar. The stones were distributed to Orym and Ashton Greymoore. Dorian Storm later gave Ashton's stone to his brother Cyrus Wyvernwind, and Orym gave his stone to Dorian. Finally, the stone held by Cyrus was transferred back to Orym before the departure of the Wyvernwind brothers.

Exandria Unlimited: Calamity
Cerrit Agrupnin had a sending stone so that his son, Kir, could contact him while he was out. Kir called him to report that his older sister Maya had snuck several drinks of liquor.