I recently encountered this video about an interesting possible reaction that could have been done by alchemists that might explain where the idea of turning lead to gold came from (in particular, wanting gold as an end product is no surprise, but why lead specifically as the source?): https://www.youtube.com/watch?v=mzsdORUPk48 It doesn't quite use period materials, for safety and convenience reasons, but I was satisfied that the described reaction could have been done, with more effort. And it does take "lead" and turn it into something that seems awfully close to gold. If you don't have an atomic theory of matter and understand how elements work, you know, I would submit that it's not even that "magical" of a thought to think that you're so close and surely there's just one or two last tweaks you can do to get there.

It's very easy to imagine that as being part of the inner mysteries of a mystery religion: https://en.wikipedia.org/wiki/Greco-Roman_mysteries

Now turning lead into gold is pure 100% unmitigated magic, because we know a lot more stuff. But if we didn't know that stuff, it would not be an obviously magical belief.