Why are diamonds a girl's best friend?

Because women make disproportionately greater parental investment in children than men do, their primary task is to discriminate between “dads” and “cads” among male suitors. How might a woman accomplish this task? How would she know which men will invest resources in her and her offspring?
As I mention in an earlier post, dads are males who are willing to invest in a woman and her offspring in the long run, and cads are those who are only looking for cheap thrills for the night and are likely to desert her after having sex. Given that women can have only so many children in their lifetimes and that they must invest much more in each child, the reproductive consequences faced by a woman for failing to discriminate between dads and cads are very large.
A good dad must possess 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 simultaneously able and willing to invest is to demand an expensive gift; only men who are capable of acquiring resources and willing to invest them can afford to give a woman expensive gifts, which are known as courtship gifts or nuptial gifts in evolutionary biology. (Yes, females of other species demand these gifts before they agree to have sex with the males.) Would any expensive gifts do? A Mercedes-Benz? A house in the suburbs?
No, these gifts will not do. A man who is intrinsically interested in luxury European cars might buy her a Mercedes. A man who is intrinsically interested in real estate might buy her a house in the suburbs. In either case, his gift is not an unequivocal and pure indicator of his general and universal willingness to invest resources in her and her offspring. The courtship gift for the purpose of screening dads from cads must not only be costly but also lack intrinsic value.
Diamonds make excellent courtship gifts from this perspective because they are simultaneously very expensive and lack intrinsic value. No man (or woman) can be inherently interested in diamonds; you cannot drive them, you cannot live in them, you cannot do anything with them. Any man who would buy diamonds for a woman must be interested in making an investment in her. Flowers, another favored gift for women, are also relatively expensive and lack intrinsic value. Of course, diamonds and flowers are beautiful, but they are beautiful precisely because they are expensive and lack intrinsic value, which is why it is mostly women who think flowers and diamonds are beautiful. Their beauty lies in their inherent uselessness; this is why Volvos and potatoes are not beautiful.
Consistent with this evolutionary psychological logic, recent analysis using game theory demonstrates that what the researchers call “extravagant” gifts -- gifts to women that are “costly but worthless” -- facilitate courtship. 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. (Once again, yes, there are such “gold diggers” among females of other species as well.) It appears that women are not the only ones who must screen their mates very carefully

By Satoshi Kanazawa