What the girl in Ambrette Town was referring to was what is called Mega Evolutions. This occurs when a certain Pokemon (i.e. Blaziken) holds a Mega Stone (i.e. Blazikenite). What will happen is when you enter a battle (wild or trainer), you will of course get the four option: Fight, Bag, Run, Pokemon. Once you click "Fight," you will see at the bottom "Mega Evolution" Click on this button and after you choose a move, your Pokemon will evolve once again. This is only a temporary evolution for battle purposes. After battle, your Pokemon will revert back to its original form. Here is a list of Pokemon that can Mega Evolve:
Mega Evolution Pokemon
- Gardevoir (Gardevoirite)
- Garchomp (Garchompite)
- Scizor (Scizorite)
- Mawile (Mawilite)
- Venusaur (Venasaurite)
- Charizard (2 Mega Evolutions) (Charizardite X/Charizardite Y)
- Blastoise (Blastoisinite)
- Tyranitar (Tyranitarite)
- Lucario (Lucarionite)
- Abomasnow (Abomasite)
- Manectric (Manectite)
- Gyarados (Gyaradosite)
- Aggron (Aggronnite)
- Houndoom (Houndoomite)
- Ampharos (Ampharosite)
- Mewtwo (2 Mega Evolutions) *(Mewtwonite X/Mewtwonite Y)
- Absol (Absolite)
- Banette (Banettite)
- Medicham (Medichamite)
- Heracross (Heracrossite)
- Pinsir (Pinsirite)
- Gengar (Gengarite)
- Alakazam (Alakazite)
- Kangaskhan (Banned from uber. Can hit twice with i.e.: Focus Punch/Drain Punch) (Kangaskhanite)
- Aerodactyl (Aerodactylite)
- Blaziken (Blazikenite)
Unreleased Mega-Evolution Pokemon
- Latios (Latiosite)
- Latias (Latiasite)
All of these Pokemon must hold their respective mega-stones if you wish for them to Mega- Evolve during battle.
NOTE: YOU CAN ONLY MEGA EVOLVE ONE POKEMON DURING BATTLE. If the mega-evolved Pokemon faints and is replaced with another Pokemon holding its respective mega-stone, the new Pokemon cannot mega evolve. If a Mega is switched out mid-battle and is switched back in, the Pokemon stays mega-evolved until the end of the battle. Any mega-evolved Pokemon that have fainted and are revived stay in their mega-evolved form until after battle.
Mega evolving a Pokemon during battle changes their stats. Take Mega Alakazam for example. In its standard form, it has a base stat total of 490. However, when it mega evolves, that base stat total shoots up to legendary status, or close to it, at 590! Its SpA shoots up from 135 to 175, and its Spd increases from 130 to 175.
Mega evolving a Pokemon (as from the example above) results in DRASTIC stat changes. This is because a mega stone brings out a Pokemons hidden/true potential.
Certain mega-stones cannot be found until postgame in X/Y. In order to find these, you must power up your mega ring in Anistair City by the sundial. You meet professor Sycamore there and he will explain. The reason for powering up your mega ring is to find the mega stones that are hidden (you can tell where they are by the sparkling aura around their "hiding spot"). These are only obtainable between 8-9 P.M.. Some stones are given/sold to you. There are three stones that you can recieve that are not retrieved in-game, but through special events.
- Blazikenite
- Latiosite (Undetermined)
- Latiasite (Undetermined)
Other's are bought (kanto starter mega stones) and those that are given to you (Gardevoirite, Ampharosite, Absolite, and others found in caves behind significant Pokemon (i.e. Mewtwo and Mewtwoite)).
I know this was probably a lot more information than you were looking for, but it doesn't hurt to have a little extra knowledge. Really hope I helped ^^
Source: Experience
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