When Pokémon Sun & Moon released, many players noted the obvious lack of Mega Stones, which were implemented in the previous generation. Post-game did feature a few of the Stones, but not all of those found in generation 6 were even obtainable. [Thanks, Serebii.]
However, those missing Mega Stones will soon be available in Sun & Moon. The Pokémon Company recently announced that the Stones will be distributed as gifts for Sun & Moon online competitions. The first Stones to be distributed will be Beedrillite and Mawilite, which will be obtainable during a competition in late February.
Pokémon Sun and Pokémon Moon are available for the Nintendo 3DS.