In fact it might just be, but not for your rock collection... for water.

>"The significance of finding opal on Mars will have advantages for future astronauts, and exploration efforts could take advantage of these widespread water resources. Opal itself is made up of predominantly two components: silica and water, with minor amounts of impurities such as iron. Since opal is not a mineral, the water is not bound as tightly within a crystal structure. This means that if you grind it down and apply heat, the opal releases its water. In a previous study, Gabriel and other Curiosity rover scientists demonstrated this exact process... a single-meter halo could house roughly one to 1.5 gallons of water in the top foot of the surface. Combined with growing evidence from satellite data that shows the presence of opal elsewhere on Mars, these resilient materials may be a great resource for future exploration activities elsewhere on Mars — that is, if opal elsewhere on Mars also retains water to the same degree as the opal in Gale Crater."

And there's no one there to mine them if you did.