If it's about rocks AND it's about space, I'm there! ... I have been going through the Opportunity archive images and it did, indeed, "just appear" ... There are several different frames of that one particular image. I download them and manipulated them several different ways, but have no clue what this rock is.
There is, however, what appears to be a "scuff mark" or "bounce mark" just to the left of the rock and slightly above it in the image. This mark appears in an area of "looser" soil between some of the desert basement rocks. If it is, indeed, a scuff mark, then the rock may have come from "screen left" of the photo.
The composition is very intriguing ... I look forward to what comes out of the past few days' research on this.
The universe is not only stranger than we imagine, it's stranger than we CAN imagine. --- JBS Haldane
Come visit me at Comanche Springs Astronomy Campus (we're on Google Maps) in Texas.