Hey, its Greg P, yes, i am on ashley's delphi name, but AH! i do have an answer!
there are typically 3 common materials...
Brass
Steel
Iron
there are also some exotic materials used too...
lexan (plastics)
titanium alloys
yes, yes made gears outa lexan...go ask team 42 about their HUGE lexan gears...
technically, you could make a gear out of anything. the results might not be pretty, although i give you props for tryin'
~Greg!~