You won't get access to Mega Evolutions until a little ways into the story. Defeat Norman, the 5th Gym Leader, and then make your way to the next town. You'll meet Steven and then head to Southern Island. Defeat Team Aqua/Magma to receive the Mega Bracelet., All of the Mega Stones that were available in X and Y can be found in Alpha Sapphire and Omega Ruby as well. Many of them require that you defeat or capture Groudon or Kyogre, which happens towards the end of the game. Some Mega Stones require you to meet other conditions. For example, Lucarionite requires that you obtain the Master Rank in all five categories of a Pokémon Contest, and then you'll need to defeat Lisia. Different stones have different requirements.


Pokémon
Mega Stone
Stone Location


Venasaur
Venasaurite
Route 119*


Charizard
Charizardite X
Charizardite Y
Fiery Path (X)*
Scorched Slab (Y)*


Blastoise
Blastoisinite
S.S. Tidal


Alakazam
Alakazite
Slateport Market


Gengar
Gengarite
Battle Resort


 Kangaskhan 
 Kangaskhanite 
Pacifidlog Town*


Pinsir
Pinsirite
Route 124


Gyarados
Gyaradosite
Route 123


Aerodactyl
Aerodactylite
Meteor Falls


Mewtwo
Mewtwonite X
Mewtwonite Y
Littleroot Town (X)*
Pokémon League (Y)


Ampharos
Ampharosite
New Mauville


Scizor
Scizorite
Petalburg Woods*


Heracross
Heracronite
Route 127


Houndoom
Houndoominite
Lavaridge Town*


Tyranitar
Tyranitarite
Jagged Pass*


Blaziken
Blazikenite
Route 120/114*


Gardevoir
Gardevoirite
Verdanturf Town*


Mawile
Mawilite
Verdanturf Town


Aggron
Aggronite
Rusturf Tunnel


Medicham
Medichamite
Mt. Pyre


Manectric
Manectite
Seaside Cycling Road


Banette
Banettite
Mt. Pyre


Absol
Absolite
Safari Zone


Garchomp
Garchompite
Fortree City (1,000 Flags)


Lucario
Lucarionite
Pokemon Contest Master Rank


Abomasnow
Abomasite
Route 123*


* You will need to have either defeated or captured Groudon (Omega Ruby) or Kyogre (Alpha Sapphire) in order for these Mega Stones to appear.
 Alpha Sapphire and Omega Ruby introduce a number of new Mega Stones for old and new Pokemon. Many of the following stones require you to complete some sort of requirement. For example, you'll receive Latiasite/Latiosite from Mom in Littleroot Town after you've met Zinnia in the game. Most of the stones below will not become available until after you've captured or defeated Kyogre or Groudon.Omega Ruby and Alpha Sapphire

Pokémon
Mega Stone
Stone Location


Beedrill
Beedrillite
Sea Mauville


Pidgeot
Pidgeotite
Rustboro City


Slowbro
Slowbronite
Shoal Cave


Steelix
Steelixite
Granite Cave


Sceptile
Sceptilite
Route 120/114


Swampert
Swampertite
Route 120/114


Sableye
Sablenite
Sootopolis City


Sharpedo
 Sharpedonite 
AS - Team Aqua Hideout
OR - Battle Resort


Camerupt
Cameruptite
AS - Battle Resort
 OR - Team Magma Hideout 


Altaria
Altarianite
Lilycove City


Glalie
Glalitite
Shoal Cave


 Salamence 
Salamencite
Meteor Falls


Metagross
Metagrossite
Pokémon League


Latias
Latiasite
Littleroot Town


Latios
Latiosite
Littleroot Town


Rayquaza
None (Story)
N/A**


Lopunny
Lopunnite
Mauville City


Gallade
Galladite
Fallarbor Town


Audino
Audinite
Battle Resort


Diancie
Diancite
Pokémon Center*


* To get Diancite, you'll need to have traded a Diancie from X or Y to your Alpha Sapphire or Omega Ruby game. You'll receive the Diancite the next time that you enter a Pokémon Center.** Rayquaza does not use a Mega Stone to Mega Evolve. Instead, it needs to learn the move Dragon Ascent.
 If you want your Pokémon to be able to Mega Evolve, it will need to be holding its Mega Stone. You can't do this during battle, so make sure that you've equipped it beforehand., This will cause your Pokémon to perform its Mega evolution. If you have multiple Pokémon that can Mega Evolve, only one is able to per battle. The Pokémon will stay in its Mega form until you have finished the battle or until it is knocked out. You can switch the Pokémon out and then back without having to go through the Mega evolution again.
Defeat Norman.
Find the Mega Stones from X and Y.
Find the new Mega Stones.
Have the correct Pokémon hold the Mega Stone.
Choose the "Mega Evolve" command during battle.
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