Ups Def and Sp.Def of evolvers. (Hold)
Eviolite boosts both defenses by 50% on Pokémon that can still evolve! Makes things like Chansey and Porygon2 have some different advantages than normal. Same with Rhydon and Rhyperior!
Only works on not-fully-evolved Pokémon. If you're running one, this is usually the right held item.
Game:Polished Crystal