They offer certified diamonds and fine jewelry. [Description from dmoz]
Blue Nile helps tech-savvy Marc Antonys bejewel their Cleopatras. The company, which offers luxury-grade jewelry online at bluenile.com, sells loose diamonds, settings, engagement rings, and other jewelry made of gold, platinum, and silver set with diamonds, pearls, emeralds, rubies, and sapphires. It is the leader in online jewelry sales (ahead of Bidz.com), offering more than 1,000 styles of jewelry, watches, and accessories. Blue Nile also operates Canadian and UK Web sites. CEO Mark Vadon, a discouraged engagement-ring shopper, and Ben Elowitz, formerly of Fatbrain.com, founded the site in 1999 as RockShop.com and, briefly, Internet Diamonds Inc., before adopting the Blue Nile brand name later that year. [Description from Hoover's]
This site reaches over 135K U.S. monthly people.