Well, it only looks pretty to you if you're into that type of thing. I don't think it would be hard to find people who don't think gold jewelry looks good. My point is that the utility is entirely dependent on the subjective experience of the user.
People in the past have traded using seeds as fiat, but they wore shiny or coloured things like shells, polished stones, polished metals.
Human vision is attracted to shiny and brightly coloured artefacts, it's not entirely subjective. This is why you can trick a child to accept a shiny coin in exchange for a high value item, or currency note, or even for a higher value coin that has less polish.