I'm not sure specifically as I haven't read this exact 'the rock short story'. But generally, in a short story about a rock, it could be about the significance of an ordinary object like a rock, perhaps its role in a certain landscape or its symbolic meaning in someone's life.
The main idea might revolve around the rock's journey. It could start with the discovery of the rock, and then how it interacts with its surroundings. Maybe it's about the rock being a constant in a changing world, or it could be a metaphor for something in a person's life, like strength or stability.
Well, in many short stories, the rock could be the central element that ties different elements together. It could be used to explore themes such as time, as the rock endures over long periods. It might also be about the relationship between humans and nature, with the rock representing the unyielding part of nature that humans come across and are affected by.