What decides a materials worth?

I've just been reading an article about a guy who gave up everything to live in a cave, he uses no money at all and lives on what he can find or is given.

One thing he said interested me though....

Suelo considers the riches of our own forage. "What if we saw gold for what it is?" he says meditatively. "Gold is pretty but virtually useless. Somebody decided it has worth, and everybody accepted this decision. The natives in the Americas thought Europeans were insane because of their lust for such a useless yellow substance."

Source:
Meet the Man Who Lives on Zero Dollars

He has a good point. All the precious materials in the world, diamonds, gold and the likes.....what dictates how much they are worth? Is it their avilability? How easy it is to retrieve it?

CX.

Value is determined by what people are willing to pay for it.

Gold is used in manufacturing computers and such, so it's not entirely useless.
Plus there is some idea that monoatomic gold can make you a god, so, there are plenty of reasons but it all depends on what someone is willing to pay.