The Crystal Dimension is where Tree Trunks is teleported in the episode "Tree Trunks" after she took a bite out of the Crystal Gem Apple. The apple was guarded by a unique Crystal Guardian, which probably means the apple is important in the Crystal Dimension. Though Tree Trunks is seen in the Crystal Dimension at the end of the episode, the dimension is unnamed until the episode "Crystals Have Power." The Crystal Dimension is inhabited by Crystal People, Rainicorns and dogs. It is filled primarily with pink crystals of varying sizes making up the whole terrain.
The connection between the normal dimension and Crystal Dimension is not fully established and the only certain method of entry is the Crystal Gem Apple. In "Crystals Have Power" Finn gets into the dimension through a portal that a crystal sent by Tree Trunks opened. How the crystal that opened the portal got into the normal world of Ooo is unclear; Finn only says he "found this crystal on our doorstep."
Sometime before the series, the Rainicorns and dogs fought over at least some territory in the Crystal Dimension in the Rainicorn-Dog Wars, as stated by Jake in "Her Parents," but it is unknown if either of them succeeded. However, it is stated that Lady Rainicorn's parents Bob Rainicorn and Ethel Rainicorn were coming from the Crystal Dimension, so it can be inferred that the Rainicorns have some current stake in the area.