Addendum: the sense in which gold isn't quite a ponzi scheme is that the initial beneficiaries are long since dead. Everybody alive has roughly the same buy-in to it, so we can trade gold among each other on equal terms.

Some original guy got rich by convincing others to take his lump of metal in exchange for goods and services. The fact that it's nice to look at helped. That happened thousands of years ago, and so we're all just living with it as a collective agreement. We could break it if we wanted to, but there's not much motivation to.