Where it isn't dust covered, the color sure looks the same as all the other basaltic rocks.

I'm going to be the pessimist and predict that it's basalt that just got fractured in an odd way. Perhaps by impact shock, another rock impacting it, sledgehammer from the heretofore secret Beagle 3 lander, whatever. Maybe some sort of fracture pattern from when the original lava cooled.

I guess my gut reaction is that it looks like "Mimi" has eroded/fractured in a different way than most of the boulders, but it looks like it's made of the same material.

I'd be happy to be proved wrong, though, and I certainly haven't examined the spectra in any more sophisticated way than looking at the color image.