This is the area where there are many crystals of varying sizes and colors that are sticking, or laying, out of the ground. It is located very near the Smoking Mountain. The gang gets over there so that Cera can get a replacement shiny stone for Tria. It is also a part of a system of ravines that prevent lava from flowing into the Great Valley during volcanic eruptions.
A volcanic eruption sends a lava flow into the Canyon of Shiny Stones during the course of the episode, so it may not be accessible any longer.
The Canyon of Shiny Stones only appeared in the episode with the same name where the main characters got there to get Cera a spare shiny stone for Tria after an accident in the beginning.