Would it be possible to let a bank generate valuables depending on the diamonds/gold intake?
Same as the factory does goods? Either diamonds deliverd or gold deliverd produces valuables ?
That would be possible with creating a new or replacement bank industry. Since you wrote in a different post that you like to play as vanilla as possible I'm not sure that is the direction where you like to go.
And for creating a new / replacement bank industry you need to have knowlegde of GRF coding or ask someone with that knowlegde for support.