It is possible to invest in many precious (e.g. silver, platinum, gold) and non-precious (e.g. titanium, copper) metals. However, investing in gold by far the most common and well known metal in which to invest.
Gold is commonly bought as a hedge against economic crisis. Often when the stock market is in decline, the price of gold will increase. Also, the price of gold has a strong correlation to the price of oil. I.e. if the price of oil increases, so too does the price of gold.
So many investors use gold as a hedge against inflation, deflation and devaluation of currency. This is because the currencies of all major countries are under pressure due to their governments deficits. As the government prints more money, the value of the currency weakens. Holding gold is one way of minimising this risk.
Check out this useful video for more information on how investing in gold can work for you: