Gold is hardly useful. All we use it for is jewelry, and plating our connectors.
The only thing that backs up gold is scarcity and the fact that people have all agreed it has value. Something that bitcoins have achieved, perfectly.
Same with diamonds.
Up to 100-ish years ago, diamonds were, to people, perceived as worthless shiny rocks. Just like any other worthless shiny rock. They only have value now because we agree they have value. Diamonds aren't even rare.
Gold is a very useful element especially for things like the development of technologies of space travel