This came up at a recent session I was in. For reasons I am unsure of, one member of the group assumed the metal flask he carried was made of aluminum. When this fact came up, another member of the group pointed out that aluminum requires an (to me, at least) surprisingly complicated process to create in a pure form. He felt that such a metal would not exist in the setting.
However, the player who owned the flask countered with the fact that there's no reason it couldn't. Specifically, if by no other means, the spell 'polymorph any object' lets people turn ore into an equal weight of the pure metal. While metals with intrinsic value are not allowed (copper, silver, gold) metals like bronze and normal iron would be. So, since aluminum isn't used as any form of currency, there's no reason it couldn't be made with this spell.
That said, the group decided that it was ultimately unimportant, and the matter was dismissed. I was personally curious, though, and decided to look through my books. From what I've found in the core rulebook, advanced player's guide, and ultimate magic (I thought a spell might require it as a material component) I was not able to find mention of it. Frequently, though, I noted that many things were denoted as just being made of 'metal.'
Can anyone cite a source for the existence of aluminum in Golarion or the Pathfinder system?