I thought it was pretty clear: a super-valuable mineral gem the size of a small hens egg they extracted from some kind of womb-like living plant or animal with a scalpel and acid. It was a kind of barter currency, so it formed the basis of a gold-rush economy with bandits, mercenaries and all.
It was pretty neat that the location was very earth-like (and obviously live-action filmed on earth, with almost no discernible cgi), but the environment felt very alien thanks to some very inventive and subtle touches.