Some people collect base metals such as copper, nickel, tin etc... and store them hoping that they will be worth more in the future. There is a phrase you may come across which is "Copper is the new Silver". Well that may be so but I would not bet the farm on it. Having said that I do have a few 1 Oz copper rounds and a couple of copper bars just in case the price really hits the roof in the next twenty years. There is nothing wrong with buying a bit of copper but, personally, I would not make it the mainstay of your portfolio. The same can be said for Zinc, Nickel, Iron, Tin etc... you can buy some if you want to but do not put a lot of money into it.
Part of the problem with base metals is that their cost per pound or kilogram is so small and places that will buy the metal from you have a large minimum delivery requirement that it makes selling them very difficult. It also makes buying them a problem as well because you have to pay a lot of money above spot price in order to acquire them. One idea is to buy them in the form of a "round" which means they look like a coin and then maybe a coin collector will purchase them sometime later.
There are people out there that will sell you exotic metals such as Indium, Osmium and others that you have never heard of. Just remember that the problem is not buying them but selling them later. They may try to sell you Indium for less than its current price and it may sound like a bargain but unless you can sell it to someone later you will have lost that money. The market for exotic metals is not an easy one to get into and is best left to the mining companies.
If you wish to invest in these metals I would recommend either buying shares in a mining company or finding an ETF that bases itself around the metal or metals you are interested in. If you want to have physical metal then do not buy too much and make sure that it is in a form that can be sold later such as a coin shape with a nice design.