The Crystal Heart is what you use to get into the Crystal Dimension, and to evolve Eevee into Crysteon! Cool, huh?
Where to find it
You find it in the Mines that reach all the way from Kanto to Kalos, where some Christions live.
How to use it
To get to the Crystal Dimension, hold it and say "Crystal Crystal Crystal" and then you'll teleport.
To Evolve Eevee , place it down on a crystal pedestal and level-up your eevee.