Gold is as much an investment as it is a luxury good.
A scarce metal, gold offers fairly competitive returns compared to other financial instruments. All currencies over time tend to lose value against gold, thus making it an ideal asset the world over. Another upside of having gold in your portfolio is that it never goes bankrupt.
Humans have traded in gold for centuries. As an inert and malleable metal, and due to its scarcity, gold emerged as a currency long before money made an emergence. This is one of the reasons why its acceptability as a financial instrument extends from local pawn shops to the largest financial institutions and governments.