An ounce of sperm
you can sometimes get them from the pokeathlon dome at the exchange counter by the national park! It should cost about 2000-3000 points!
Coal is the remains of ancient plant life. It does not incorporate precious metals to any significant degree.
Niobium is a rare metal, good additive for stainless, but not precious -it is grey
Rhodium is pretty rare and very expensive. More rare than platinum.