Yeah I know this isn't something I would normally discuss, but I have a friend looking for an engagement ring so I offered to help him out. I started surfing the web a bit because I thought there must be a site out there by this day and age to use and came up with this one:
Pretty impressive site. You can browse through by almost any means neccessary, be it by size, price, carat, color, or many others.
You probably don't want your girlfriend or wife to find this site because she will fall in love and you will be buying diamonds left and right. You could probably earn some points with her though if you introduced her to the Diamond Builder on the site. Put it this way. If there is a game you want to watch, tell her about this site and you will have an uninterupted one :D.