I never understood the whole concept of a woman wanting jewelry from a man, especially diamonds, until I read the book Why Beautiful People Have More Daughters.
In the book, two evolutionary psychologists explain why people do what they do. Why are diamonds a girl's best friend? The authors conclude that women have to discriminate between "dads" and "cads" among male suitors. In order to find the guy that will stay with her and help her with children, she looks for two qualities: "the ability to acquire and accumulate resources, and the willingness to invest them in her and her children." A good way to screen for men who are both willing and able to invest is to demand an expensive gift--known as a courtship gift or nuptial gift in evolutionary biology. Not just any expensive gift will do. A Mercedes or house does not usually fill the bill--for these might have intrinsic value to the man if he likes European cars or is interested in real estate. A courtship gift, according to the book, must be costly and lack intrinsic value and be useless.
Diamonds and flowers are beautiful and their beauty lies in their inherent uselessness; "this is why Volvos and potatoes are not beautiful." So guys, apparently, gifts that are "costly but worthless"--facilitate courtship, according to a recent study mentioned in the book.
"The researchers note that such extravagant gifts have the added benefit for men of deterring 'gold diggers,' women who promise to mate in exchange for a gift but then desert without mating after receiving it."
So, it seems that diamonds serve as a litmus test for how a woman feels about you. I still have trouble with the idea of "demanding" expensive gifts but it all makes a bit more sense to me now.