Yes, because they are shiny, but so is aluminum foil. Gold and diamonds are expensive moreso because they are both rare, hard to get to, and nobody's making any more.
Or, because you and the rest have been convinced that they're rare and hard to get. Truth is, there's billions and kazillions of them. They come out of the ground if you dig in the right spot. Heck, there's more gold in Saudi Arabia than water.