I think if you replace the word 'plain' with 'classic' it is a better perspective. I'm sure he isn't thinking plain as in boring or uninteresting..but more simple and elegant and classic. Sometimes guys don't know how to say what they mean to make it sound good.
Do you think he might be overwhelmed by the options? At Christmas I sent my BIL some links for my SIL (she wanted gelish kit). I spelled it out very clearly and simply...Order this, this and this. he was still overwhelmed and didn't get it for her, lol.
The risk with presenting her with just a stone is that she may fall in love with a setting that is beyond the budget. Giving her a classic yet simple set gives the option to upgrade with nicer bands as finances allow.