"Well, all the currencies I can think of are not investments."
What a bizarre thing to say. People have always bet on exchange rates, switching from sterling to dollars etc.
People bet at roulette wheels, that doesn't make roulette bets investments. The fact that city types trade currencies does not make currency trades investments.
My definition of investment is something with a positive expected return. Shares give you profits, bonds (loans, debt) pay interest, property pays rent. Those thing are all investments.
Gold, over hundreds/thousands of years, has on average maintened its value in real terms, so it has been a useful store of value, a hedge against inflation, but it doesn't have a return. In fact it costs you money to own, as you have pay to store and protect it. It is sometimes held by investors for its diversification benefits, but that still doesn't make it an investment.
I suggest you look at the contents of a typical pension companies generic investment fund, the default fund that gets used by people who don't have specific ideas about how they want to invest. You will find that they own shares, bonds and maybe property. You probably won't find them owning gold, or hedge funds seeking profit from currency trading.