It depends on which kind of rock Geodude is made of and what cave he is in.

The Alolan Geodude line has magnetite crystals in it, but otherwise appears to be made of the same kind of material that Geodude is. This is actually helpful for figuring out what Geodude is made of, since it implies that Geodude is made up of a type of rock that sometimes is mixed with magnetite. And as it turns out, it's often mixed into sand or sand-derived sedimentary rocks. That would explain Graveler's name (gravel is basically larger sand, right?). So we can assume that the Geodude line is based on sandstone.

Now, it would be unreasonable to go through every cave in the Pokemon series and try to guess what kind of cave they are. However, if we look up famous caves in Japan from around the same regions as the Pokemon regions that inspired them, we can see limestone caves in Kanto and Hoenn, lava plume caves in Johto around IRL Mt. Fuji. Limestone is primarily calcite and stuff, whereas sandstone is more quartz-heavy, and lava plumes aren't even sedimentary. So if caves in classic Pokemon games are similar to their IRL counterparts, Geodude is made of very different material.

TLDR: Caves are not flesh and flesh is not caves.

The real existential dread of Geodude should be how he's literally nothing more than a rock with fists, who evolves into a bigger rock with more fists, and yet isn't a Fighting type.