Emeralds are the green variety of the mineral beryl, whose more-common blue counterparts are known as aquamarines. Frequently flawed and riddled with inclusions, emeralds are somewhat brittle gemstones, and they are not as hard as rubies and sapphires, let alone diamonds. For these reasons and others, finished emeralds are often given an emerald cut, which describes a step-cut of parallel facets that wrap around the crown (top) and pavilion (bottom) of the stone. In fact, in an emerald cut, the gem’s corners are even removed to keep them from chipping away later.

Words like cloudy, sleepy, and soft are often used to describe an emerald’s brilliance. In fact, from a monetary standpoint, emeralds are judged less on their clarity than their color. Unlike sapphires and rubies, emeralds cannot be heat treated to bring out their color, but the stones are routinely oiled to fill cracks and mask flaws, which has a similar effect.

The oiling of emeralds dates to the time of the ancient Greeks, so it's hardly a new practice. More controversial is a newer technique in which a polymer or resin, with a refract...ive index equal to that of an emerald’s, is forced under pressure into flawed or cracked stones. Unscrupulous dealers will traffic in stones whose polymer has also been dyed, masking the artificial ingredient’s appearance, but some resins are known to yellow with age. Buyers of vintage emeralds in necklaces, bracelets, earrings, and rings should ask to see a report about their stones from the Gemological Institute of America (GIA) or a similar third party.