Diamonds are not girls best friends, but they are De Beers best friends. Well, being a geologists wife I have been informed how boring and over priced diamonds really are. It's all in the marketing (and bribing the major producers not to sell their raw merchandise), what makes those pieces of carbon so expensive.
My (very simple) engament ring has a rectangular Tourmaline in it, it is a very dark green, almost black stone, we chose it because we both liked it immediately. Not that I wear it daily though (not a jewelry person here), I just like to look at it (and put it on when visiting grandparents)