From Bulbapedia, the community-driven Pokémon encyclopedia.
An example of a Crystal Pokémon
Crystal Pokémon (Japanese: ニュートラルポケモン Neutral Pokémon) are a type of Pokémon in the Pokémon Trading Card Game. They are considered by fans to be the e-Series equivalent of the Shining Pokémon previously introduced in the later Neo expansions, Neo Revelation and Neo Destiny. The Pokémon featured in each of the cards are not alternately colored however.
Introduced in Aquapolis, Crystal Pokémon are identified by being Colorless but focus on three particular types of Energy to use in its attacks. All Crystal Pokémon have the Poké-Body Crystal Type, in which the type of the Pokémon changes from Colorless to one of three specified types if the corresponding Energy card is placed on it, for one turn. These types are also emphasized on the card design, for example, Celebi's Poké-Body and attacks focus on Grass, Water and Psychic; the colors green, blue and purple are highlighted in the card background.
While Crystal Pokémon were not bound by the 1 per-deck rule like Shining Pokémon, they still featured several Energy for each attack and were quite costly for the effects they provided.
In Japan, these cards were released three at a time over the course of the last three e-Card expansions, but with the hurried release of the English sets and their condensation, Skyridge featured six. In both languages, all Crystal Pokémon are secret cards. Crystal Pokémon were retired at the end of the e-Card Series.
Pokémon that have appeared as Crystal Pokémon