You seem not to understand the nature of diamonds… or economics.
“Natural” brilliant diamonds are valuable because they are relatively scarce and the sources are generally very well controlled by a diamond cartel headed by the DeBeers Corporation and (this is important, too) they are heavily promoted and advertised as “a girl’s best friend”, and “the only stone to have in ‘meaningful’ jewelry”. So DeBeers gets to set the price, and they get your girlfriend / wife / mistress to whine that she has to have a big, brilliant diamond.
Other than brilliants, industrial quality diamonds (with all of the hardness properties, but not the brilliant view) are relatively inexpensive.
Gold, aside from being a relatively scarce element which has historically been used for coinage and “store of value” for millennia, also has a great many industrial (as well as medical / dental) uses and therefore has demand ranging from coinage and jewelry to industrial, medical and electronic applications. And we haven’t figured out yet how to “manufacture” elements (other than the newest elements of the Periodic Table, which are sometimes little more than “theoretical” elements with no known use—or appearance in nature).