Tips on buying diamonds in New York City

A diamond is probably the oldest thing you’ll ever own—maybe as old as 3 billion years and believed to be the hardest thing nature has produced.

If you’re planning to buy one, there are several things you should know.

Value is determined by a diamond’s cut, color, clarity and weight, all of which should be graded and certified by an independent agency such as the Gemological Institute of America, which established the 4 C’s standard for rating gems.

Experts consider the cut most important because it influences the stone’s brilliance. The whiter the diamond’s color, the better and more sparkly it will be. Virtually all diamonds have occlusions or trapped crystals, affecting clarity, but those with imperfections undetectable without a 10-power magnifier are most desirable. And, of course, weight is important.

After comparing prices at several establishments and finally settling on a dealer, find out:

• How long he’s been in business and if he’s had gemological training and belongs to one of the major professional organizations such as the Jewelers Board of Trade or World Federation of Diamond Bourses.

•Are there any unresolved Better Business Bureau and/or state consumer-affairs agency complaints against the business?

•What’s the guarantee and refund policy? (Some merchants permit only merchandise exchanges, not cash refunds.)

•Ask if the diamond has been treated to remove imperfections and/or tinted to improve its color. (Federal law requires dealers to inform prospective buyers if it has been. Treated gems may enhance glitter and hide imperfections but reduce value.)

•Is it a “conflict” diamond—one that funded an insurgency or African warlord?

•Will a detailed receipt, appraisal and GIA certificate substantiating its rating be provided?

•And will the full cost of the diamond be refunded if an independent appraisal is less than the dealer’s, and what’s the deadline for returning the gem?

Finally, and most important, be prepared to patiently negotiate the price.

—S.L.