Provavelmente 1d6 dmg.
Do feitiço: link
The ranged attack is made as if you had thrown the item unless it's ammunition for a weapon
Arremessar uma pedra é uma arma improvisada de longo alcance. Para descobrir o dano, você encontra um objeto similar da tabela de armas e usa seu dano.
Sometimes objects not crafted to be weapons nonetheless see use in combat. Because such objects are not designed for this use, any creature that uses an improvised weapon in combat is considered to be nonproficient with it and takes a –4 penalty on attack rolls made with that object. To determine the size category and appropriate damage for an improvised weapon, compare its relative size and damage potential to the weapon list to find a reasonable match. An improvised weapon scores a threat on a natural roll of 20 and deals double damage on a critical hit. An improvised thrown weapon has a range increment of 10 feet.
Neste caso, acho que a rocha provavelmente causa dano semelhante a um taco que causa 1d6 de dano.