Surprised nobody has mentioned this one: tell her you'd rather start saving for the kids' college fund. That oughta have at least a little tug on her heart strings....even if you have your fingers crossed behind your back.

If you're serious about just buying a precious metal that you'll never wear and have in the back of your head that you can sell some day, just buy a brick of platinum or something. OR, broaden your search beyond a Rolex...