Gloom's branchedevolutionary line as demonstrated in the anime: both of Gloom's potential evolutions require an evolutionary stone.
An evolutionary stone (Japanese: 進化の石Shinka no ishi) is any one of ten stone-like items that radiate a mysterious energy that causes some species of Pokémon to evolve. An eleventh stone, the Everstone, is different in that radiates a type of energy that prevents evolution in the holder regardless of its species.
Evolutionary stones may be used at any time, and cause instant evolution in the Pokémon they are used on, which cannot be cancelled. With the exception of the Oval Stone and Everstone, which must be held for their effects to take place, all evolutionary stones are applied directly to the Pokémon. All stones that cause evolution in a Pokémon, including the Oval Stone, are consumed upon that Pokémon's evolution.
The use of a stone is seen by many to be "forced" evolution, especially by the main characters of the anime, as the Pokémon is not in charge of its own transformation. Many Pokémon that result from a stone-based evolution also have vastly different level-up learnsets than their pre-evolutionary forms, with several learning no moves after evolution at all.
An item to be held by a Pokémon. The Pokémon holding this peculiar stone is prevented from evolving.
In the anime
File:Large Moon Stone.jpgAsh, Misty, Brock, and Seymour watch as Clefairy dance about a giant Moon Stone.
Evolutionary stones are showcased somewhat rarely in the anime, as only a relative few Pokémon are able to evolve by using them. Despite this, they were among the earliest of items to have been showcased in the anime, appearing as early as Clefairy and the Moon Stone, where a rather large Moon Stone appeared deep within Mt. Moon. It was being worshiped by a group of Clefairy who lived there, and was soon stolen by Team Rocket and ended up being blown up. The shards from it, however, rained down on the Clefairy and caused some of them to evolve.
The second appearance of an evolutionary stone occurred in The Electric Shock Showdown, where the Thunderstone was introduced. This was likewise the first opportunity that a Pokémon belonging to a member of the main cast was given to evolve by stone. Ash's Pikachu, however, refused the offer, intending to beat Lt. Surge's Raichu as a Pikachu.
The remaining Generation I evolutionary stones made their debut in The Battling Eevee Brothers, as part of a collection held by the Eevee brothers. Additionally, two of the Eevee brothers offered a Fire Stone and Thunderstone to Brock and Ash, respectively, to evolve their Vulpix and Pikachu, though both refused.
A forest that appeared in The March of the Exeggutor Squad was a reported hotbed of Leaf Stones, and the radiation from these stones in the forest was so strong as to induce evolution in Melvin's Exeggcute, which proceeded to cause chaos by hypnotizing a horde of other Exeggutor.
A fake Leaf Stone, made by Team Rocket, appeared in Make Room for Gloom, where the discovery of its nature as a fake was central to the episode. A real Leaf Stone finally appeared at the very end of Pikachu's Rescue Adventure. With it, the Exeggcute that had been following Misty's Togepi around evolved into Exeggutor.
Fire Stones played an important role in The Stolen Stones, where they were intended to be delivered to a stadium to be used as a prize for competitors there.
The Water Stone appears in Once in Mawile and is used by Brock's Lombre to evolve into Ludicolo. This is the first time that a main character's Pokémon evolves via an evolutionary stone on-screen, although it is likely that James's Weepinbell was evolved by way of a Leaf Stone prior to The Breeding Center Secret.
In the Yellow arc chapter, Yellow finds a Leaf Stone in Vermilion Harbor, but the Fire, Thunder, and Water Stones are missing. Later, it is revealed Giovanni gave them to Red so he could evolve his Eevee.
Only four Pokémon that evolve by stone can evolve using another method: Poliwhirl, Eevee, Kirlia, and Snorunt.
Only Template:Type2 Pokémon evolve using the Leaf Stone and Sun Stone, and both Pokémon that evolve using the Sun Stone evolve into pure Grass-types.
The Oval Stone is the only evolutionary stone that must be held by the Pokémon, instead of used directly on it, to induce evolution. Thus, it works more like a Razor Claw or Razor Fang in functionality.
Of all the Pokémon that evolve by evolutionary stone, only Happiny is able to evolve further.
All Pokémon that evolve with the Fire Stone and Thunderstone are in the Ground Egg Group, all the Pokémon that evolve with the Leaf and Sun Stones are in the Plant Egg Group, and all the Pokémon that evolve with the Shiny Stone are in the Fairy Egg Group.
Both Pokémon that evolve with Shiny Stone evolve into Pokémon with a red right appendage and a blue left appendage.
Togekiss is the only Pokémon that evolves from its pre-evolution by way of a stone that shares no moves in its level-up learnset with its pre-evolved form.