Metal-rich rings, metallic asteroids. Mine. Don't think you can actually buy it on commodity markets, it's one of those that just vanishes out of the spacefaring economy as soon as you unload it.
Which it would, it's an absurdly useful metal with a very large number of applications. Densest naturally occurring element, part of the platinum group, probably a choke point in the making of all kinds of things that are purchasable on the commodity market.