The whole concept of traditional diamonds is repulsive - a completely useless thing, bought for way too much money, mined in the most oppressive conditions. The idea that the ideal way to start a relationship is to spend thousands on a worthless stone, and that this waste of money is somehow romantic... makes me ill.
It's mindboggling to me that a woman would find wasting money on diamonds attractive (same goes for super-expensive weddings). You could use the money to put some money down on a house, or save for a future child's college fund, etc. That strikes me as more romantic, too, in a more practical sense - you're planning your life together rather than throwing cash down the drain.
On the other hand, artificial (cubic zirconium) diamonds are kinda cool, in a 'demonstrates the power of science/engineering' kind of way.