The Gemological Institute of America (GIA) ratings are the most widely recognized ratings for diamonds.

At the top of the scale is D, the best. D, E and F rarely are seen in an average jewelry store. G, H, I are good colors. The higher the letter, the more yellow or silver -- it's beginning to pick up color.

Clarity ratings begin with "flawless," which is very rare, and then "internally flawless." "VVS," very, very slight flaw, and "VS," very slight, are only visible under a 10-power magnification microscope. "SI" is slightly imperfect.

"Get everything that somebody represents to you in writing," says jeweler/appraiser James Rosenheim. "If they don't use GIA ratings, they should be able to translate it into GIA ratings.

"Good color, eye clean and the classic, 'our best grade,' just won't do."

* For complaints and free publications, "Guides for the Jewelry Industry," "Guides for the Watch Industry," "Buying Diamonds," "Buying Gold Jewelry," "Gemstone Investing," write Bureau of Consumer Protection, Federal Trade Commission, Washington, D.C. 20580.

* For complaints, write The Jewelers Vigilance Committee, 1180 Avenue of the Americas, New York, N.Y. 10036.

* For free information on appraising: the American Society of Appraisers, P.O. Box 17265, Washington, D.C. 20041.