Pearls?
Not sure how mineral precipitation could take place within the "host" rock. But it does seem to explain why they're laying around on the surface instead of being buried or weathered away.
WAG- Fossilized gas bubbles.
When the layered material was still "mushy", pockets of gas (not a clue as to which gas or why) formed spherical "molds". Over time, the parent clay hardened and the bubbles filled in.
Viola, Martian marbles.