There are two cardinal rules for antique jewelry: Know what you have and know how to care for it. Not as easy as it sounds, however.

A woman recently sought an appraisal for a 12-plus-carat antique emerald ring, estimated to be

Round, brilliant cut 2.78-carat diamond.

worth more than $500,000. She was stunned when the appraiser said the stone was cracked and therefore had lost much of its value. The Chubb Group of Insurance Companies, unwilling to accept the jeweler’s verdict, presented the ring to the Gemological Institute of America for inspection. It determined that the stone was not cracked at all. Rather, it had been fracture-treated, a process now used frequently in which the cavities in fissure-prone softer gems are filled with oil under high pressure. The emerald had been given a clear appearance and the procedure had even afforded protection against breakage. But over time the oil had deteriorated and leaked out, so the stone appeared to have been compromised.

“All that was required was that the right people look at it, treat it and handle it,” says Janece White, assistant vice president of Chubb and a GIA-accredited jewelry professional. “So the stone, which was almost written off, was re-enhanced and restored to its original value for a couple of thousand dollars. And while rubies and sapphires are worth less when enhanced because they can be found in a perfect natural state, emeralds often have imperfections.”

The worth of antique jewelry is tied to three factors: the cost of the materials in the piece, how popular is the style currently and its provenance. When in doubt, always consult a well-credentialed appraiser with expertise in antique jewelry.

Gem expert Robert Aretz was suspicious of a blue brooch brought to him for valuation with a handwritten note saying “Mother’s Aquamarine Pin.” The color bothered him. Testing revealed that the stones were diamonds, not aquamarines. The GIA confirmed that the diamonds’ color was a natural, intense blue, and the proper identity significantly changed the monetary value (and, possibly, the sentimental value). Aretz points out that items handed down through the family can be misidentified, and the mere fact that estate jewelry is found in a safety deposit box does not necessarily speak to its esteem or value.

Antique jewelry must be accorded the respect that its age has earned. “Modern jewelers, no matter how skilled, do not understand antique jewelry and can destroy the value of a piece that, properly restored, would maintain its value,” says Robin Kaplan of Arthur Guy Kaplan. “Hallmarks can be removed, enamel cracked, and hard solder used—all of which devalue a piece of good older jewelry. Excessive heat can melt filigree, crack stones or discolor gold. Today, many antique repair professionals use lasers to fix antique jewels, which are particularly effective because no heat is used.”