While obviously you can't really transmute something like Lead into Gold (as far as I know), is there any real-life mineral or element that could be used to artificially create Gold from whatever its composite elements are? If so, what kinds of processes would you have to run the ingredients through to make Gold?
Changing Element A into Element B requires changing the number of protons in each atom of Element A. There is NO chemical process that can affect the number of protons in any atom.
Chemical processes can be used to concentrate or dilute gold atoms mixed among others. Chemical processes can bond or separate gold atoms locked in compounds. But no chemical process can change another element into gold.
...assuming your alchemist is honest, of course. Any charlatan worthy of the name can whip up a convincing transmutation swindle.
While it is impossible to transmute any element into another by chemical means, it is entirely possible to do it via nuclear means.
In fact, Glenn T. Seaborg in 1980 was able to transmute several thousand atoms of bismuth into gold, however at a net loss economically speaking (the cost of equipment, energy, and bismuth was greater than the value of the gold.)
Given your response to existing answers, I have a potentially showy solution for your would-be alchemist:
Calavarite (naturally occurring gold telluride)
It's uncommon in real life, but could be unusually common for whatever reason in the vicinity of wherever this feat is to take place.
Your would-be alchemist/wizard is dropping fancy stones into vitriol, as one does, to see what happens. To his shock, a gleam of gold starts to form as the liquid turns red. He has discovered the Philosopher's Stone!
(He has not. But it's a good way to get more research money from the local royalty!)