NEW YORK — Few purchases are more mystifying for first-time buyers than fine jewelry. There's more information out there than ever — from jewelers' Web sites and online forums to nearly ubiquitous grading reports from independent labs. But buying expensive gems and precious metals is still largely a matter of trust between you and the jeweler.

First, educate yourself on the basics. For diamonds, that means the four Cs: cut, color, clarity and carat weight. For gold, platinum and silver, it means purity.

You can find helpful information on these fundamentals from the Federal Trade Commission (www.ftc.gov) and the Better Business Bureau (www.bbb.org). The Gemological Institute of America, the most prominent diamond-grading agency, provides tutorials on buying diamonds and colored gems at (www.gia.edu).

"It's less of a blind purchase than it used to be," says Jay Mednikow, president of 115-year-old Mednikow Jewelers in Memphis and Atlanta. "But a jeweler who knows what he's doing can take advantage of you if he wants to."

Thus, there is still no substitute for a reliable dealer with an established reputation. Many jewelers are GIA-certified gemologists and display their credentials prominently.

For diamonds, Mednikow recommends buying only those with grading certificates from GIA, the American Gem Society or another independent laboratory. If a jeweler says he can offer you an uncertified diamond at a discount, tell him you'll pay to have it analyzed since the cost should be only $50 to $300 depending on the size of the stone. Read warranty and return policies carefully and make sure all guarantees are written on your sales receipt — it's your legal contract.

You may have a hard time distinguishing between slight variations in color and clarity, but still trust your own eyes.

Mednikow recommends holding diamonds with a pair of tweezers over your finger or against a white background and under lights of different types and varying brightness. With shapes other than round-cut, which has standard specifications, and with colored gems, you will have to rely much more on the jeweler's expertise.

If you are buying a colored stone such as a ruby, sapphire or diamond, ask if it has been "treated" to enhance the color. Some processes are routine, like heating for sapphires and rubies and oiling for emeralds, but others are temporary or undesirable.

Up to half the gold jewelry sold in the U.S. bears a false karat rating, says Mednikow. Choosing a reliable merchant is your only insurance, although national retailers like Zales and Sears are diligent about the purity of their gold.