The Eviolite (Japanese: しんかのきせきEvolution Stone) is a type of held item introduced in Generation V. It boosts the Defense and Special Defense of a Pokémon that holds it if the holder is not fully evolved.
The Eviolite only affects Pokémon if their form and species has non-zero value in their "personal data", (internal data that determines the basic traits of any given Pokémon), for an Evolution method, regardless of whether that Evolution method is legitimately accessible.
While this functions as expected for most species/forms, there are some notable unusual cases such as instances where an Evolution method "exists" but isn't accessible, as well as some hardcoded exclusions:
If only the regional form of a species can evolve, only the regional form is affected by the Eviolite.[1]
Meltan is not affected by the Eviolite, because it cannot evolve in the core series games, only capable of evolving while in Pokémon GO.
It does not affect Cosplay Pikachu, Pikachu in a cap, or Eternal Flower Floette, as those specific forms have no Evolution method coded, even if the base species is capable of evolving.
It does affect maleCombee and Salandit, despite only female Combee/Salandit being capable of Evolution, (as they are the same species and form as their female counterparts and therefore share the "non-zero Evolution value" in their personal data).
It still affects certain Pokémon that are capable of Evolution, even if it is/was not currently possible to meet the requirements for their Evolution.[3]
In Pokémon Scarlet and Violet, it affects Ursaring and Stantler despite their Evolution methods being inaccessible within Scarlet and Violet. HisuianQwilfish was also similarly affected for the same reason before The Indigo Disk added its evolutionary method into Overqwil.
Eviolite was introduced as a Pokémon Tool card in the Pokémon Trading Card Game during the English Black & White Series (the Japanese BW Era) in the Noble Victories expansion. When this Trainer card is attached to a Basic Pokémon, any damage done to this Pokémon by attacks is reduced by 20, after applying Weakness and Resistance.
Trivia
Eviolite's name may be derived from evolution and the -ite suffix found in the names of many minerals.
Eviolite's Japanese name is a pun as しんかのきせき (Shinka no Kiseki) can mean either "Evolution Stone" or "Evolution Miracle."