An emerald is the brilliant green version of the gemstone beryl. People have prized these vivid gems since antiquity. On Pliny's advice, the Roman emperor Nero commissioned the shaping of an exceptionally clear emerald through which the ruler could "refresh and restore" his sight as he watched gladiators in combat. Modern emerald aficionados can choose a number of less expensive alternatives to the precious gemstone, but first they must be able to spot a genuine emerald. A jeweller first determines the authenticity of the emerald, then ascertains whether the gem is natural or lab-created.
Look at the stone's colour. Both natural and lab-created emeralds range in hue from a pale but vivid green to a deep true green. Stones with a distinct yellowish or chartreuse cast are not emeralds, but are most likely peridots or green garnets.
Note any fire that the gem displays. Fire, or what gemologists term as dispersion, refers to the scintillant (flashes of light) spectral hues that a stone shows under white light. Diamonds, for example, have a good deal of fire. Natural emeralds have low dispersion and should show little fire. Flashy green gems are probably cubic zirconia.
Examine the stone edge as well as the top facet to spot a doublet. Manufacturers can sandwich a thin layer of paler emerald between pieces of cut glass using deep green epoxy to mimic a higher-quality emerald. Looking at the gem's sides reveals the layers to detect these fake stones.
Look at the stone through a jeweller's loupe. If the edges of the facets seem worn, the stone is probably not a genuine emerald, but is emerald-coloured glass. Natural and lab-created emeralds have a hardness of 7.5 to 8 on the Mohs scale, higher than that of glass. (A diamond is a 10.) Relatively soft glass has a hardness of 5.5 and loses its sharp edge quickly with time and use.
Note the price of the gem. Although natural emeralds and lab-created emeralds contain the same constituent elements, naturally formed stones cost hundreds or thousands of dollars per carat, depending on clarity and colour.
Examine the gem with unaided eyes. Natural emerald stones frequently contain inclusions of liquid, gas or mineral solids that give the gem a cloudy or dusty appearance. Larger emeralds are less likely to exhibit good clarity.
Compare the gem's colour to others, if possible. Lab-created emeralds uniformly exhibit the vivid green hue that jewellers prize most in natural emeralds, while naturally formed emeralds vary in colour. If a given stone matches others in the jewellery case perfectly, it is probably lab created.
Look at the stone through a jeweller's loupe to examine its inclusions. Jewellers have a term for the unique pattern of inclusions that each stone contains, called the emerald's jardin, from the French word garden. Each stone's jardin of bubbles, feathers and fissures is as unique as a fingerprint, and only natural emeralds feature a jardin.
Emeralds represent the month of May in birthstone lore. Fissures in natural emeralds could soak up chemical cleaners, so clean natural mined emeralds with a soft cloth.
A natural emerald's characteristic inclusions can make the stone fragile, so avoid cleaning natural emeralds in ultrasonic cleaners. Most emeralds sold with a descriptor before the gem's name, such as night emerald or bohemian emerald, are substitutes or synthetic gems.