When it comes to buying an engagement ring online, Blue Nile and James Allen are the two biggest jewelers and, in my opinion, the very best.
Blue Nile offers a huge selection of diamonds and settings, and you can design your own engagement ring at a great price. They have a solid reputation, and they are the most popular online jeweler.
James Allen also offers a wide selection of diamonds and settings, including fancy color diamonds and gemstones. Their prices are slightly higher than Blue Nile, but they offer a lot of great features and service that justify the cost.
The truth is that you will be able to get a great engagement ring from either retailer, but I personally think James Allen offers a bit more, which makes for a better shopping expereince.
James Allen displayes every diamond in 360 degrees. Each diamond is magnified in high definition, so you can see exactly the diamond you will be getting. Even though Blue Nile offers similar diamonds with grading certificates, since you cant see them, you can't be sure exactly what you're getting. When I'm shopping for something expensive, I don't want to deal with uncertainty.
Also, Blue Nile only offer 30-day returns, whereas James Allen offers 60-day returns. The extra month really provides better peace of mind.
Although James Allen sells for a bit higher than Blue Nile, I think the ability to see diamonds and have an extra month to return your ring justfies the difference.
I hope this helps.