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.
Gloom's
branched evolutionary line as demonstrated in the anime: both of Gloom's potential evolutions require an evolutionary stone.
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.
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.
Ash won a Sun Stone in the Bug-Catching Contest during The Bug Stops Here, later using it to evolve a Sunkern in Moving Pictures.
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.
Both the Leaf Stone and Sun Stone appear in Whichever Way the Wind Blows as a demonstration of the branch in Gloom's evolutionary line.
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 Stopped in the Name of Love, Dawn used an Everstone on her Piplup to prevent it from evolving.
Professor Oak gave a Water Stone to a wild Lombre in A Faux Oak Finish!, while a Dusk Stone was central to the plot of Try for the Family Stone!.
Strangely, evolutionary stones do not seem to be required for evolution in the anime as they are in the games. James's second Weepinbell evolved without a Leaf Stone in Here's Lookin' At You, Elekid, though this may just be an example of anime physics.
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In the Red, Green & Blue arc of Pokémon Adventures, a Moon Stone is first mentioned by Professor Oak in Gyarados Splashes In! as a rock with a moon-shaped inscription that boosts a Pokémon's power immensely. In the next round, Raging Rhydon, Team Rocket searches for one in Mt. Moon, however it is Red who finds it. Later on, Red uses this Moon Stone to evolve Green's Clefairy. Red's Poliwhirl later evolves into a Poliwrath with the help of a Water Stone when he falls in the water.
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.