Iron-nickel meteorites are incredibly hard
Nick has already mentioned this but I'll elaborate. I carry a small iron-nickel meteorite in my pocket as a fidget rock. It is incredibly hard (like case hardened steel). I've tried to scratch glass with it but not succeeded (it is not diamond hard). In an environment where it can't rust, I imagine my meteorite would last almost forever. However when the regolith of Meridiani was moist with sulfuric acid, that Martian iron-nickel meteorite would have quickly corroded. The prestine condition of that meteorite is pretty good evidence that Meridiani has been bone dry for a long time.
Gary