Most people invest in either gold or silver. Some prefers the titanium too. Investing on these precious metals is a smart move because I believe that the value of these will increase over the years.
One of the other metals that one can invest on is the Rhodium bars or Rhodium Coins. They may not be a very common investment but this precious metal is worth a lot.
Like any other investment, be sure to familiarize yourself about them before engaging. It is always a plus factor if you know what investment you want to put your money on. There are plenty of available resources in order to learn more about metal investing.