Something like this actually happened. There was a super rare element that was shiny and light weight and incredibly rare. They made the crown jewels of France out of it so the King could show off the wealth of the country to everyone by having this unique and rare element in his crown.

Then they discovered a chemical process to extract this incredibly rare element from bauxite.

Now we make pop cans out of it. Yep, aluminum was at one time a precious metal.

also the global gold market would collapse into dust

Cheap and plentiful gold would be a boon for a lot of applications that require resistance to water corrosion, so even after the initial collapse, its price would slowly creep up again.

Lead (II) iodide is the closest we have

Technically, isn't it possible to turn lead into gold already? Its just an insanely expensive task (the whole adding protons to atoms) with no real benefit (since the result will be way more expensive than even regular gold).