In our junior high creative writing class we we are taught "man vs. nature". This leads many to believe that a mountain, or a wave, or even a rock can function like an Antagonist.
Unless someone represents the rock or speaks for the rock, it won't come across as a character, but rather a force of nature. No one interacts with the rock which, as Chris Huntley (Dramatica co-creator) puts it:
is essential for an objective character – no push and pull between forces; no act, counter, dodge, attack, etc.
So, they need a voice.
directed action with a particular purpose -- to separate [them] from "stuff happens"
A rock can be an Antagonist, if the Author attributes some reactive responsibility to the inanimate object. They need to Prevent. And they need to Reconsider.