Millenia ago, aliens came to earth to mine our gold. Being more technically advanced than us, they had real applications for it, and needed it in large amounts.

In the process of mining, they enslaved us, took advantage of us. They made us call them gods and they lorded over us like kings. Gold for them was something to kill for. We didn't know why, but we knew these rulers sure loved it.

when they left, they left a power vacuum behind which was quickly filled by opportunists who took over the role of being 'gods'. In this position, they also mined gold - something that was seen as the rulers' due. Except, these new kings didn't really know what to do with it, so they made jewelry and adorned their crap with it.

It has been considered valuable ever since.