Gate stone

A gate stone (also gatestone, or capitalized Gate Stone or Gatestone) is a magical item that facilitates travel between distant locations. By activating a gate stone, a person can travel to the location of its twin gate stone as long as they are on the same plane of existence.

History
Two gate stones were gifted to Vox Machina by Lillith Daturai during Winter's Crest in Whitestone. Percy handed one to Cassandra, partly as a token of trust, partly to provide each of them with a means of communication or escape in case of emergency.

After the Chroma Conclave's attack on Emon, Keyleth used Transport via Plants to bring refugees from Greyskull Keep to Whitestone. Percival de Rolo joined the first group to ask Cassandra de Rolo for help with the refugees, then used a gate stone for the first time to return to Greyskull Keep. When activated, the stones gave off a soft, bluish-purple light and shook; as soon as the teleportation occurred, the stones became white-hot and gave off a scent of burnt cedar. Percy gave the gate stones to Vex and Vax.

As the party gave chase to Umbrasyl, Pike handed off her gate stone to Keyleth. Keyleth tried to use it to teleport in earth elemental form to Vax, who had been inside Umbrasyl's body. However, by the time the gate stone activated a minute later, Vax had escaped the high-flying dragon and was falling at terminal velocity, so Keyleth appeared in the sky and fell as well. Confident that her earth elemental form would absorb all the damage from falling 200 feet, she lit her hands on fire (with the spark stone power), curled up, and acted like a meteor.

Keyleth used a gate stone to collect Pike Trickfoot for the fight against Vorugal.

Having just returned from the Elemental Plane of Fire with the aasimar twin boys, Hunin and Kyor, the party found themselves in the middle of a pitched battle: Thordak's forces were on the verge of overrunning Fort Daxio. On Percy's advice, Keyleth gave one of the gate stones to the twins, and they told the boys to run and hide, and the party would find them after the battle. True to their word, Vex used the twinned stone to retrieve them when it was safe.

and
In preparation for a ritual to resurrect Scanlan, Vex'ahlia enlisted the help of Eskil Ryndarien to teleport to Westruun, then gave Kaylie a gate stone so she could go to Whitestone and help persuade Scanlan's soul to return to his body.

When Scanlan decided to leave Vox Machina, Vex gave him one of the gate stones.

Vax had an idea for how to escape from the Mentiri that involved the gate stones, but was informed that the stones had to be on the same plane, and the other one was still in Scanlan's possession on the Material Plane.

Percy confided in Vex'ahlia that he wanted the gate stone and Earring of Whisper back from Scanlan, and that he was thinking of collecting them. He felt betrayed by Scanlan, especially given how useful the bard could have been in the situations the party had faced since he left.

Vax developed another plan using the gate stones: his sister would use oil of etherealness to make her way down to the buried Sword of Kas and, when the oil wore off, retrieve the blade and use the gate stone to return. They did not think this plan through very well, however, and although they got the sword they also revealed their location to Vecna and, under attack from the Briarwoods, had to flee to the Feywild.