Since the time of Cleopatra, emeralds have epitomized the of color in green gemstones. It would be easy to question this statement if all one had seen of emeralds were the commercial, (and poorer,) quality stones which abound on home shopping networks and in some jewelry stores. A fine emerald, though, is a truly breathtaking sight and is well deserving of its placement in the traditional "big four" along with sapphire, ruby and diamond. Emerald is the birthstone for May and for commemorating the 20th and 35th wedding anniversaries.
... The center of world emerald mining is in South America with Colombia and Brazil as major producers. The African mines that supplied Cleopatra's passion have long since been played out. However, today the African continent is second only to South America in production, with mines in Zambia, Zimbabwe, Madagascar and Nigeria.
... Each of these world locales typically produces a certain color, size and clarity -- so much so that the term "Colombian" emerald has often been enthusiastically used to describe vivid, slightly bluish green stones of medium to medium dark color, no matter what their actual geographic origin. Likewise, emeralds of lighter color are sometimes called "Brazilian", even if they were mined in Africa. The USA and Japan together purchase more than 75% of the world's cut emeralds.
... Emerald, by definition, is a medium or darker green to blue green beryl, in which the green color is derived from impurities of Chromium, Vanadium, or a combination of both. Before 1963 the definition was limited to Chromium containing stones, but the discovery of a large deposit of Vanadium colored stones in Brazil led to modification.
... Varying amounts of iron will affect the color as well, with more atoms of this impurity increasing the bluish tones. In a situation similar to that which exists with the boundary between pink sapphire and ruby; there are chromium colored stones of light to medium light green color which are sometimes sold as emerald, but which are more correctly considered green beryl. Geological conditions were right, it seems, in Colombia to produce exactly the slightly bluish green shade and strong saturation that make stones from that locale the epitome of the variety.
... Emeralds are considered a "Type III" gemstone by GIA which means that they are virtually always included to one degree or another. Because of this designation, a clarity grade of "very slightly included" for example, refers to the normal range for emeralds, not for all gemstones. Well over 90% of the emeralds in commerce have been treated to minimize the appearance of the inclusions.
... The industry practice for treatment, (and that which is considered "standard" by AGTA,) is "oiling". This term refers to the practice of immersing emeralds in a colorless oil or resin. Often this is done using a vacuum chamber to assist penetration. Non-standard treatments go beyond this to using green colored oils and hardened, epoxy-like resins.
... These treatments dramatically improve the appearance of the gems, but necessitate special care in cleaning and setting. Steam cleaners, solvents and ultrasonics can remove the oils, making inclusions which had barely been visible stand out in sharp relief. Luckily, it is possible to have emeralds re-oiled.
... The inevitable inclusions are more than a aesthetic consideration, as they can reduce the structural integrity of the gem as well. Beryls, in general, are good jewelry stones, with a hardness of up to 8 and no troublesome cleavages. Because of the inclusions, emeralds are generally more fragile than other beryls and must be treated more gently.
... Emerald imitations often encountered in the marketplace include: glass, YAG, synthetic spinel triplets, green cubic zirconia, and beryl triplets. Within the last fifty years two major processes have been developed to produce "lab created" emeralds, or synthetics. If you've seen and priced man-made emeralds you might have wondered why they are so costly compared to CZs or some types of synthetic sapphires. Both the flux and the hydrothermal methods of production require costly equipment and are energy intensive. They take a long to time produce and have a low yield of cuttable gems.
... Some of the first lab created emeralds on the market weren't convincing because they were so clean, but the sophistication of today's consumer has led to a trend toward more naturally included looking synthetics. Although this improves their acceptability, it does make it a little more difficult for gemologists and appraisers to prove natural origin. Fortunately, there are signs, particularly regarding the types of inclusions in a gem, which can conclusively verify natural versus synthetic origin.

... Like many stones, the per carat price of fine quality emerald escalates rapidly with size. For example, a recent price guide lists a fine quality, 3 carat Colombian stone as six times more valuable than three equivalent quality 1 carat stones.
... Value factors hinge largely on color with nuances of saturation and hue affecting price to a significant degree. The most desirable color is a slightly bluish green in a medium dark tone with strong to vivid saturation. Clarity is important, but inclusions are tolerated more in this variety than virtually any other gem. Top quality, unenhanced stones, (with certification,) can bring as much as 50% more in price than treated stones of the same size, color and clarity.

Facts about Emeralds

CHEMISTRY Be₃Al₂Si₆)O₁₈ + Cr
CRYSTALLOGRAPHY Hexagonal
REFRACTIVE INDEX ~ 1.57 - 1.59, varies with source.
HARDNESS 7.5 - 8
SPECIFIC GRAVITY 2.68 - 2.78
CLEAVAGE Indistinct
HEAT SENSITIVE No

WEARABILITY* Poor to Good, depending on the integrity of the gem.
SPECIAL CARE INSTRUCTIONS Emeralds usually have internal fracutres, so clean with warm or room temperature soap and water. Avoid wearing gem where it will get rough treatment.

ENHANCEMENTS Oiling, common. (Oils and epoxies are used to fill fractures, which reduces their visibility.