There was a thread on this on the space science forum last week. I'd post a link but the "earlier" link isn't showing up.

Nick and JMS had a lively exchange about what sort of crystalline material it might be. JMS suggested aragonite as one possibility, or a hexagonal form of gypsum ("ram's horn") -- I think that was the most liked.

Don't see anything that suggests recent "oozing" in that photo. But yeah, does like an awful lot like some sort of crystalized deposit, some sort of sulfate perhaps.

If it was corundum, as suggested on the Mars Society board, that certainly would have been noticed as a very difficult RAT -- probably would have damaged the cutting edges. Very hard stuff. Instead, all the evidence suggests it was fairly soft stuff. Makes anything gemstone-like rather unlikely (but makes soft sulfates or carbonates quite plausible).

Sometimes the relatively modest level of speculation here doesn't indicate lack of imagination, but rather the possession of expertise and scientific skepticism.