IIRC there are frequent dust storms on Mars. Could it be that particles rolled along the ground by a strong wind eventually wore down into a spherical shape?
Geodes are naturally occurring crystal formations within certain types of rock (on earth). They're not alive, and never were, but their shape is often spherical.
Spheres are a VERY rare thing to occur naturally in minerals. Crystal habits don't lend themselves to forming spheres. Nature has a few, but they aren't common here on Earth.