The phrase "on rock" typically refers to something that is resting or positioned on top of a solid, immovable substance. There are a number of synonyms that can be used to convey a similar meaning, depending on the context and the intended level of specificity. Some possibilities might include "perched on stone," "balanced on granite," "resting on bedrock," "sitting on a boulder," "anchored on a cliff," or "fixed on a crag." Each of these terms suggests a different type of surface or environment that the object in question may be situated on, providing a more vivid and precise picture of the scene.