I would stay away from platinum. It has very limited industrial use and has not had the intrinsic value that gold and silver have had. The history behind platinum is pretty interesting as at first Spaniards found it to be nuisance metal when they were looking for silver in South America.
If you want to go outside of gold/silver, then I might recommend copper. If we didn't have copper, there'd be no TV, electricity, cell phones, computers, refrigerators, or pennies as we know it.