The problem is the shape, not just the material. As a layman, I'm not used to rocks having that shape, except in very rare circumstances but in those cases "abnormal" shapes are usually found in clusters (Grand Canyon etc). This is a seemingly-solitaire rock of abnormal material AND abnormal shape, which is possible, but almost as unlikely as random probe debris.
Just to put a bit of context here, you should visit Death Valley. It is the strangest and most wonderful place you can walk around and find amazing mineral deposits, geologic formations, and generally odd sorts of things.
Mars has had during its life a very active volcanic system, these systems imply magma, and magma implies melting and cooling of basalt. 'Shiny' objects in volcanic fields are typically obsidian or some other form of basaltic glass. Erosion by wind of a piece of obsidian can carve it into a pretty bizarre shape.
So all said and done, the geology is fascinating. And one of the differences between having a rover up there and having humans up there was that this would have been a 15 minute news story with humans, "Oh look, shiny!", "What is it?", photo, photo, chip, chip, "Some sort of <analysis result>, what is over where you are?"
Now its 'next day' at best, and 'in a month' if there is a long list of things to do before you get a break at worst. Sigh.
