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.