There are some beautiful pieces of jewelry being made from titanium and niobium, both of which are corrosion-proof, non-allergenic, lightweight, and reasonably priced. Platinum has snob appeal, but titanium or niobium seem like better choices for techno-geek types. And if you are bored with the colour, you can anodize titanium or niobium, causing the metal to change hue in striking ways.
If a woman really loves you, she will cherish the moment of your proposal, even if all you offer her is an onion ring as a memento.