Holy misinformation, Batman!
Aluminum is a metal, it is also a pure element. It's manufactured via electrolytic processing from an ore called bauxite. This might be why some people think it's man-made. By the same logic, so are many metals - they have to be smelted from their ore forms, and the impurities are skimmed to leave pure, molten metal.
The periodic table is not divided into 'gases and metals', it's divided into metals and nonmetals. The nonmetals exist on the right side of the 'staircase' which is to the left of Boron, Silicon, Arsenic, Tellurium, Astatine, and (theoretically) Ununoctium.
At any rate, aluminum is a metal and it would most likely be detected by an airport metal detector - the damn thing picked up my package of GUM (Trident SuperPak with the metal foil...) and I almost had to STRIP before they let me pass. Fuckers...
-M