95% of Earth's opals come from Australia's Red Centre. After the Eromanga sea disappeared a few million years ago, it left vast tracts of Australian soil stained red by iron oxide, and the geologically rapid drying also formed an abundance of opals.
The surface of Mars went through a very similar process when the planet lost its oceans. So the Red Centre of Australia is very Mars-like, and there may be opals hiding under the Martian surface!
I may have set a short story in an opal mining town on Mars for exactly this reason.
Of course the question on going to mars to extract opal and exploit resources there is going to have to be discussed more widely at some point. Do we as humans even have a right to claim Mars as our own to do this.
But that debate is for another time.
@zleap People really need to move past the immediate urge to "exploit" things.