The Sisters Brothers (2018)
FAQAdd to FAQ (Coming Soon)
Warm had a background as a chemist, and had developed a formula that allowed for gold sitting in a riverbed to glow when contacted by it.
The film is not as detailed on what the chemical is specifically, but the book makes it clear that Warm has spent some time developing and perfecting it, and that it is a concoction of some sort as opposed to a single chemical. Edit (Coming Soon)