The Rock Planet is a scrapped location in Serious Sam: The First Encounter.
The Rock Planet is presumptively be a planet with a lot of rocks and large structures made out of them. The Stone Elemental would likely be the planet's elemental enemy. No screenshots of this area exist, so it is impossible to determine exactly what happened in it.
It would consist of one level; Rock. After completing it, the player would be sent to Space Crusader.
The Rock Planet, like all of the other post-Egypt story arc locations of Serious Sam: The First Encounter, were cut before the game came out.