According to the Rulebook (V7)
Rocks must be at least 1 foot in diameter. When hurled two handed, rocks do two points of damage. This weapon is only avaliable to barbarians.
Legal requirments for Rocks.
A blow by any part of a boulder or rock counts as a hit; so all parts of these weapons need to be Strike-Legal (V7). The tips of all projectile weapons must not be able to fit through a 2.25 inch ring. Projectiles, with the exception of javelins, may not have solid cores. Materials you may use for projectile cores include foam, sweatshirt material, and other soft, non-granular and non-rigid materials. Pennies, batteries, sand, and the like may never be used as cores. Rocks must have a legal Cover with no exposed tape.