Following this discussion, we now have a custom close reason that says,
Please review our rock identification guidelines to provide the missing information so that your question is both answerable and useful to new users.
With a link to the guidelines. It's hidden under the "off-topic" menu, which is unfortunate, but AIUI that's the only part we can customise. But lots of identification questions are still being closed with "unclear what you're asking". My guess is that the diligent people who do all this close-flagging aren't aware of the new close reason.
So, it exists. Please use it! :-)