Value is in the eye of the beholder really
Gold should not be as valuable as it is, it's not even that rare. Not sure if this is true or not but I heard somewhere that there is enough gold in the world's oceans that if it could be extracted you could give everyone on Earth 7lbs each. Is just that nobody has found a cost effective way of extracting it yet. Besides being hard to tarnish and a great conductor of electricity it really has no traits that are beneficial to humans.
Diamonds, again, not that rare, just a hard Rock