White diamonds have long been the stone of choice for engagement rings. Now colored diamonds are beginning to catch up. Last month the Gemological Institute of America, one of the largest independent diamond-grading labs, reported a twofold increase in demand for the gems since 1999. And online merchant BlueNile.com just launched its first collection of colored stones. The growing interest stems in part from the wave of celebrities (most recently, Brittany Murphy and Rebecca Romijn) getting engaged with them.

Most colored diamonds, because of their rarity, cost more than white ones. But some, including "cognac" (brown) and mixed colors like orangey-yellow, are still attractive but cost less.

Make sure to do your research before shopping: the rules for buying colored diamonds are far different from those for buying white ones. "Color is king, everything else is secondary," says Fred Cuellar, author of "How to Buy a Diamond." Don't worry if you see minor inclusions, or imperfections, when you hold your stone under a microscope. And be sure to examine the stone loose, because some settings can make a diamond's color appear richer than it is. Go for a color grade no lower than "fancy." (The grades are "light fancy," "fancy," "fancy intent" and, the most expensive, "fancy vivid.") Last, make sure the stone has a lab report from the GIA or the American Gem Society Laboratories that is no older than six months. That way, you can be sure the diamond is natural and hasn't been irradiated to enrich its color. Now you're ready to make a brilliant choice.

—Anna Kuchment