I think it is valuable to tag a question with both rocks and identification-request. Among the most upvoted identification-request questions, some have nothing to do with rocks:
Although the tag wiki seems to assume identification requests are always about rocks, this is true for a majority but not for all cases. I think the identification requests listed above are on-topic for our site, but clearly the guidelines on how to ask identification request questions are only relevant for rocks and minerals (I don't know the difference.).
Certainly the tagging can use some maintenance. Most identification-request questions are about rocks or minerals, and most rocks or minerals questions are identification requests, but neither implies the other. Therefore, I think we should use the tags independently.