I do not recommend choosing a diamond. Diamonds are both less pretty than and more expensive than moissanite; if you have the budget for a diamond, you can get better for cheaper with moissanite. The exception is if you know for a fact the recipient is a natural stone chauvinist, which doesn't sound like your situation at all (you basically can't get natural moissanite). Bonus: moissanite is from SPACE.
If you are unwilling to consult her in advance on her taste in rocks, the safe choice is a gold-band solitaire with a round brilliant cut rock set in prongs. More expensive, more interesting, and also pretty safe is a "past present future" setting with three rocks, matching if you want to be conservative about it. I'm not sure what the conventional alloy for gold-looking jewelry that needs to not deform with use is, but if it looks like yellow gold and anyone makes a point of telling you how many carats it is, it's probably good.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.