BEFORE YOU SHOP for diamonds, the first step is to do some homework. The Gemological Institute of America, or GIA, has a Web site, gia.edu, with lots of information on diamond buying, including a primer on the 4 Cs--cut, color, clarity, and carat.

If you're buying a diamond over one carat, look for one with a GIA or American Gem Society (AGS) report--paperwork attesting that the stone is a certain color, clarity, and weight. Examine the diamond through a jeweler's loupe to make sure it's the one in the report--you can tell by matching up flaws.

Stones are also certified by the European Gem Lab, but experts don't consider the grading as rigorous.

The "C" that's hardest for a consumer to compare is cut. The better a diamond is cut, the more it sparkles. If two diamonds of the same carat, color, and clarity vary greatly in price, the less expensive one may be an "off cut" or "off make" with poor proportions and symmetry.

"You'd be surprised by how many people come in and say, 'Wow, did I get a deal on this diamond.' I say, 'I hope you got a discount for this diamond being off-make,' " appraiser Edward Czarnetzky says.

AGS is the only lab that grades cut, assigning a number from 0 (which it calls ideal) to 10. GIA does not grade cut, but plans to start next year. The only way to really judge cut is to look at lots of good diamonds side by side to see which look more brilliant to you.

Some stores liberally use the term "ideal" or use their own labels to describe diamond cuts. But there's no universal agreement about what proportions make an ideal cut--there's more than one way to cut a beautiful stone. While some stores call an AGS-graded "triple-zero" stone ideal, others use ideal as a marketing term.

"There's stuff every day being touted as ideal that is not," longtime jeweler Jerry Root says.

Web sites are a good way to learn more about diamonds and to get an idea of prices. Many stores may try to match an Internet price. In fact, Washington is a great place to buy diamonds.

"This is almost like a little West 47th Street, or diamond district," master gemologist appraiser Martin Fuller says. "In the Washington area, we have more diamond sellers than most metropolitan areas. That's kept markups down."

Every fine jeweler sells diamonds, including those listed in the previous sections. Here are more places to look:

Bill Scherlag, Tysons Corner; 703-288-3893. Scherlag runs a bare-bones operation, with no inventory. Meet by appointment to tell him the kind of diamond you're looking for and he'll work through suppliers to find it--for less than you'd pay in a store. He'll refer you to craftsmen who make custom pieces.

Direct Jewelry Outlet, 101 E. Broad St., Falls Church, 703-534-2666; 1411 St. Germain Dr., Centreville, 703-266-8200; djo.com. The settings may not be the finest you'll find, but the prices on loose diamonds are very competitive. In Falls Church, ask for Alain Planche.

Five Star Jewelers, 9655 Lee Hwy., Fairfax, 703-385-3300; 4diamond.com; 5765-V Burke Center Pkwy., Burke, 703-239-1300; fivestarjewelers.net. These no-frills shops offers some of area's best prices on diamonds and gold chains.

IGS, 444 Maple Ave. E., Vienna; 703-281-1888; igsdiamonds.com. Ed Czarnetzky Jr. keeps prices low by keeping a small inventory. If he doesn't have the stone you want, he'll get it within a day or two. An on-site goldsmith/platinumsmith can do custom mountings.

Princess Jewelers, 529 Maple Ave. W., Vienna, 703-255-5050; and 11520-G Rockville Pike, Rockville, 301-231-6060; theprincessjewelers.com. A decent selection of stones and settings at good prices.

Protea Diamonds, 2499 N. Harrison St., Arlington; 703-536-9822; proteadiamonds.com. A discount operation with prices similar to what you find online. By appointment only.

Washington Diamond, 1243 W. Broad St., Falls Church; 703-536-3600; washingtondiamond.com. A good selection of wedding and engagement rings and a knowledgeable staff. 

Web sites. Diamonds have become such a commodity they're now easily bought online. Solid sites include niceice.com, dirtcheapdiamonds.com, goodoldgold.com, usacerteddiamonds.com, whiteflash.com, bluenile.com, and dsourceva.com.

Don't want to check all these sites? Type in your diamond specs on pricescope.com, and it pulls together prices offered online.