Wasn't it explicitly stated somewhere in X/Y that Mega Stones were the result of stones being irradiated with fallout from the Ultimate Weapon? Maybe I missed something but I don't recall anything about landed meteors, I assume the bit about them falling to earth was simply because the explosion from the Weapon being fired launched them into the atmosphere the way a real-world high-yield explosion or impact event will launch ejecta into the atmosphere.
We already know radiation in the Pokemon universe is basically just a magic force that causes evolution (and probably not cancer, otherwise my OR/AS character running around with dozens of Mega Stones in his backpack at any given time is dead as fuck), so it seems like as good a nebulous, hand-wavey explanation as any other.
We already know radiation in the Pokemon universe is basically just a magic force that causes evolution (and probably not cancer, otherwise my OR/AS character running around with dozens of Mega Stones in his backpack at any given time is dead as fuck), so it seems like as good a nebulous, hand-wavey explanation as any other.