Many minerals will form spherical crystals under the correct circumstances, and spherical shapes are often formed by geological processes. You can find objects like this in the canyon lands of Utah and Arizona.

The Mars mission geologists call them "blueberries" and they've been finding them for years. On a planet with only rocks to look at, you're going to find lots of odd things that are perfectly natural but look weird as we have very different conditions here on Earth.

No big mystery, but really cool.