The Celestial Pearl is an item added by Thaumic Tinkerer. It can be bound to a Celestial Gateway by shift right clicking on one. Shift right clicking in the air will remove the bound location. It can also be put in the player's hotbar for them to use the Wand Focus: Celestial Recall. These pearls can also be named in an Anvil so it appears when using a Celestial Gateway for easy labeling.
In order for locations to be set in the Gateway, they must be marked with Celestial Pearls. These items can be shift-right clicked into a Celestial Gateway to mark its location (note, shift-right click on air to clear the location).
Placing the pearls inside a Celestial Gateway opens access to teleport to the one it's bound to. This teleport is one way, and two pearls must be used for a 2 way transport.
Naming these pearls with use of an Anvil will make the names appear in the Gateway's GUI, for easy labeling.