The elixer for iron would be of course water in order to detect the presence of rust, but I imagine things like vinegar could be used to see if copper assay samples could be tarnished and a positive test would be making the sample turn green. You'd also need to test for lead content if assaying for tin, because I've heard that a lead content rendered many tin sources unfeasible.
In that case.....

Ooh, how about we have different elixers to prospect for different minerals!!?
(It's bad thing when Cookie gets excited. People log out in disgust.)
But on the other hand, how about we get a clear guide for intermediate level players first, so they don't fail to find a mine due to the unaccountable oversight of not experimenting with manufacturing perfume?