I think I'd lean towards adding a tag for the rocks that show up a fair bit, as someone could have an interest in that specific rock type, and numerous questions show that it is has usefulness to having them as a tag. Whereas rarer used rocks don't show that interest/need.
Novice users may not know what category a specific rock type is, and very well may not think to search for it under the category. So it may mean more unnecessary work.
Plus having it as the terms now given encourages questions that aren't about the rocks but the processes. I could be fine for leaving it as is too, though do see the benefit of branching into separate rock categories, thereby leaving rock less of a generic grabbag... it's just whether someone wants to go to the work to do that. Collecting them into their wider type categories would just turn those terms into grabbags the same, and I'd envision would swamp out any questions on the processes that may be of more interest to people currently following the category, than many questions that may subsequently fall into the tag group without having much direct connection to the topic.