From Bulbapedia, the community-driven Pokémon encyclopedia.
A crystal figure is a special type of figure from the Pokémon Trading Figure Game. It is identical to a normal figure except that it is made out of a shiny, transparent material, similar in appearance to crystal, and that the base is not transparent at all. The EX rare figures are the only ones which have crystal variations of them.
They can be compared to Holofoils in the Pokémon Trading Card Game. However, unlike Holofoil cards, they are not numbered as separate figures from their normal counterparts.
List of crystal figures