Gemmologists refer to the Four Cs of Connoisseurship when grading coloured gemstones. In order of importance these are: Colour, Cut, Clarity and Crystal. Colour considers a stone’s hue, saturation and tone. Cut describes the percentage of brilliance in the stone when viewed face up as well as its proportions - length, depth and width. Clarity defines a gemstone’s inclusions or flaws, which will add to a stone’s value when used for identification purposes. Inclusions are only called flaws when their presence negatively affects a gemstone’s beauty and durability. Crystal refers to the stone’s transparency; a good, well cut stone will appear to glow.
When it comes to emeralds, however, Crystal becomes the second most important criterion after Colour. An emerald’s inclusions are considered part of the stone’s character, and can be used to confirm it is a natural gemstone. Commonly referred to as jardin - the French word for garden - an emerald’s inclusions bear a strong resemblance to foliage. Inclusions in emeralds are sometimes referred to as the stone’s birthmarks.
An emerald should display a fine, dark green hue with a high transparency. Although inclusions are tolerated, they should not affect a stone’s transparency. Too many inclusions will give an emerald a cloudy, opaque appearance, while too few will render the stone transparent.
The Hardness of an Emerald
According to the Mohs Scale of mineral hardness, emeralds range from 7.5 to 8. Diamonds, the world’s hardest substance, are rated 10. Although emeralds are relatively hard stones, their inclusions make them a sensitive, fragile gemstone. In the 17th century jewellers developed a rectangular step cut known as the Emerald Cut, still commonly used for emeralds today because it offers the best setting for displaying this gemstone’s unique colour.
Oiling an Emerald
Traders from India originally used clear oil, paraffin and wax to treat their emerald gemstones. Today most emeralds are treated with oil, which enhances the stone’s clarity and makes surface fissures almost totally invisible. Clear oil is applied to the emerald’s fissures, and a reputable gemmologist will be able to confirm if a stone has been oiled. Oiling is not usually permanent, and the process may need to be repeated every few years.
Although the application of colourless oil is acceptable, the use of green oil is considered fraudulent. Today the clarity of emeralds is improved with the use of not only oils and waxes but also a clear resin called Opticon, which lasts longer than oiling. Opticon is an acceptable practice, unless a green dye is used to improve the stone’s colour.
How an Emerald is Formed
Emeralds are members of the beryl group of minerals. While most stones in this group are colourless, the emerald’s green fire is formed over centuries through the presence of chromium. Minute traces of chromium, iron and vanadium grow very slowly within metamorphic rocks, their size restricted by the host rock. This means large emeralds are rare, and extremely valuable.
The rarity and value of emeralds has led to the development of synthetic emeralds. A thin layer of emerald placed on a colourless beryl stone under strict laboratory conditions will grow at a rate of 1 millimetre per month. Because it can be difficult to differentiate between the natural and the synthetic stone, a gemmologist should be approached to apply a series of tests to confirm an emerald’s authenticity.
World’s Top Emerald-Producing Countries
Today the world’s best quality emeralds are produced in Columbia. Arabic Emperor Jehangir’s cup made of pure emerald is displayed at New York’s Museum of Natural History. The cup is placed next to the “Patricia”, which at 632 carats is one of the largest Colombian emerald crystals ever discovered. Colombia’s Bank of Bogata has a collection of five emerald crystals, with weights ranging from 220 carats to 1,796 carats. In addition to Colombia, emeralds are also found in Afghanistan, Australia, Brazil, India, Madagascar, Pakistan, Russia, South Africa, Zambia, Zimbabwe and in North Carolina in the United States of America.
Cleaning an Emerald
Emerald jewellery should be cleaned with a soft, dry cloth. Avoid using hot soapy water, household chemicals and general jewellery cleaners which can damage or remove the oil used to treat the surface fissures.
The magnificent green colour of the emerald conveys not only nature, but also harmony and life. It is said the human eye will never tire of the colour green, which is never monotonous but always refreshing and vivid. While the colour green does change between natural and artificial light, emerald green retains its vibrant hue at all times and in all situations.
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