Crystal Dimension

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. However, this is not revealed until the episode "Crystals Have Power." The Crystal Dimension seems to only be inhabited by Crystal Guardians and possibly Rainicorns.

Sometime before the series, the Rainicorns and Dogs fought over at least some territory in the Crystal Dimension in a war called the Rainicorn-Dog Wars, but it is unknown if either of them succeeded. However, it is stated that Lady Rainicorn's parents Bob Rainicorn & Ethel Rainicorn were coming from the Crystal Dimension, so it can be inferred that the Rainicorns have some current stake in the area.

Trivia
In "The Limit" There is a crystal monster that may be from the crystal dimention. This is neather conformed nor denied.

Briefly, Tree Trunks is shown out floating in space, and Bob and Ethel Rainicorn show Finn and Jake a picture of a dog in anaveator's jaket, proving to that at some extent, the crystal dimention has areas not enclosed like the rooms shown in "Crystals have power".

It is unknown if the crystal dimention will be shown again in anything more than camos.