When buying a diamond ring, learn the four C's

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This publicity photo provided by BlueNile.com shows a model wearing a diamond ring. About 11 percent of diamond rings are bought online, and Blue Nile is one of the largest online-only retailers of diamond rings, with 325,000 couples having purchased engagement rings from the site. Shoppers can easily compare the carat weight, color, clarity and cut of various stones, and the site has a 30-day return policy as well. (AP Photo/Blue Nile)