Is it possible to implement a "collection" kind of achievement in Game Center achievements?
Let's say you design a game with a lot of gems that the player picks up. The gems come in ten different types, and some gems are rarer than other. Only when you have picked up at least one gem of all ten types, the "Collector" achievement will be 100% completed/unlocked.
There is no such feature to Game Center achievements (at this point, who knows about future).