Rocks are Boffering's equivalent of dodgeballs.
The rules on Rocks vary greatly from group to group. In some groups, they're treated as normal projectiles, damaging any limb they hit etc. Several societies however, treat them as a Headshots-only weapon, meaning they can only kill or wound via a hit to the head.
Rocks are generally made from the scraps of spare foam left over from making other weapons. The scraps are simply gathered into a large ball, and taped until there's no foam showing.