That was my first impression of those, that they looked like limestone. I have studied literally tons of Texas Hill country limestone, and that is what the Opportunity rocks look like. I don’t think they are limestone, but I bet they formed in a similar fashion as limestone.

As for the sphere, I still have not ruled out that they were formed by a biological process, because I can not find a better non-biological explanation for them. And also because limestone is full of fossils, and if these rocks formed in a similar manor, and life was around at the time, they should also contain fossils.