The value of everything is based on the individual's subjectivity; and I suppose you could call that "psychology". The individual's decision-making process -- according to his own perceived needs and wants -- is the entire basis of economics.
There is absolutely no physics involved, though the modern "mainstream" economists (think Krugman, Bernanke, etc.) have tried mightily to pretend otherwise.
Put it another way... Imagine all the pine wood in the US disappearing tonight: we’d be screwed. Imagine all the steel disappearing; we’d be screwed. Imagine all the concrete or all the corn disappearing; same thing, we’d be screwed. But if all the gold were to disappear, I don’t see how that would hurt anything.