The vivid green of a good natural emerald is highly prized and a high quality emerald can be worth more per carat than a diamond. Anu Manchanda, senior gemmologist for AnchorCert Gem Lab, considers the most important factors relating to emeralds.
Emeralds have been sought after for centuries. In the 1st century BC they were discovered in the Mines of Cleopatra, between the Nile and the Red Sea in southern Egypt. Emerald became prized as the symbol of immortality, and was used in religious and formal ceremonies as well as being used as an antidote for poisons and to prevent epilepsy in children. Some also believed that by holding an emerald under your tongue, you could see the future.
The world of gemstones was transformed in the 16th century when the Spanish conquistadores discovered the Colombian emerald mines and introduced Europeans to the superb quality Colombian emeralds. Today 60 per cent of the world’s emeralds come from the Muzo mines in Colombia, with Brazil, Zambia, Zimbabwe, Pakistan, Afghanistan and Madagascar accounting for the other 40 per cent of the world’s supply.
Natural emeralds form in metamorphic or sedimentary rocks as elongated six-sided prismatic crystals, when beryllium, aluminium, silicon, oxygen and one or more trace elements come together in the right geological environment. Emerald crystals have nearly uniform physical properties with insignificant cleavages. The ‘emerald cut’ has been developed to suit the natural characteristics of an emerald and its brittle nature. It is a step-cut with a large square or rectangular table and truncated corners, resulting in minimal weight loss from the rough stone.
For coloured stone professionals emeralds are, most of all, about colour. The most desirable emerald colours are bluish green to green with strong to vivid saturation and medium to medium-dark tone. As a natural product no two stones will be exactly the same. Wide variations in the hue tone and saturation of the emerald crystal are caused by naturally occurring trace elements such as chromium, vanadium and iron, and depend on the presence and absence of each element and their relative amounts.
Almost all emeralds have eye-visible inclusions. Eye-clean emeralds are especially valuable as they are so rare and the presence of inclusions in emeralds is usually accepted by the trade and the consumer. Emeralds can be lightly, moderately and heavily included. Commonly found inclusions are crystals, fractures, needles, fingerprints, growth tubes, liquid inclusions, and two- or three-phase inclusions. These are often described as mossy-looking or garden-like. Sometimes the French term ‘jardin’, meaning garden, is used in the trade to describe these inclusions. A gemmologist presented with what is apparently a near perfect emerald with an intense hue and good clarity will automatically check its authenticity and look for evidence of treatments.
Natural emerald has been treated by man for almost as long as they have been cut. The eye-visible, surface reaching fractures in emeralds become less visible by soaking them in certain oils. The jewellery trade accepts the practice of oil filling as long as clear oil is used. The refractive index (RI) of the used filler is close to the RI of emerald and the fracture becomes less visible as the filler seeps in. A variety of oils have been found to be suitable: cedarwood, castor, coconut, corn, joban, linseed, lubricating mineral, neat’s-foot, olive, palm, peanut, rapeseed, soy, and tung oils. Oiling is not considered permanent and can change with time, heat or solvents. An experienced treater can re-oil the stone and this may need to be done every two to five years.
Sometimes coloured oil is used to alter the colour of the emerald – this practice is not accepted by the trade without disclosure. In recent years resin has been introduced as a more permanent filler for emeralds. This may be natural or manmade and hardener may be added to improve the durability of the stone. It is estimated that more than 90 per cent of the emeralds now on the market have fractures filled with oils or resins. Untreated emerald can be up to 50 per cent more valuable than treated emerald of apparently the same quality.
With good quality natural emeralds attracting a premium price there are inevitably many imitations. The most prevalent are coloured glass, but these are usually easily identified as imitations by gas bubbles, concave surfaces and rounded facet junctions. Other natural, synthetic, or assembled materials used to imitate emerald are synthetic spinel triplet, green synthetic cubic zirconia, green YAG and beryl triplets. These may be more difficult to identify without laboratory equipment.
The first synthetic or ‘created’ emerald crystal was synthesised in 1848 by a French chemist using the flux process. The first marketable synthetic crystal was made by American Carroll Chatham in 1935. Johann Lechleitner synthesised emerald hydrothermally in 1960. Later many producers started producing synthetic emerald by both processes. Natural emerald and flux and hydrothermal emerald can all appear the same to the unaided eye. By using a microscope the difference can be revealed in growth zoning, types of inclusions, and pattern of stress cracks. Generally synthetic emeralds have fewer fractures and higher clarity than natural emeralds.
Caring for emeralds
Emeralds are brittle and need careful handling. Setting of emeralds needs special experience, as they are far less resilient than a diamond. Warm water, mild detergent and a soft brush should be used to clean emerald jewellery. Ultrasonic and steam cleaners are not safe as they can remove the oils, increase fractures and break the emeralds. Emerald jewellery should be stored in jewel boxes separately from diamonds and other harder gems.
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