The Dorgesh-kaan sphere teleports the player to Dorgesh-Kaan. They can be purchased and used upon completion of the Death to the Dorgeshuun quest.
Oldak, the cave goblin professor, will construct these spheres for the player if they provide him with 2 law runes and 1 molten glass (which can be noted). Players with at least 52 Thieving can also steal the spheres from his chest.
Unlike most other forms of teleportation, spheres do not teleport players to one set point; instead they place one at random in various locations of the area they are aligned to.
It will not work past level 20 Wilderness.
For an exhaustive list of all known sources for this item, see here