Emeralds belong to the mineral family known as beryl: several different varieties of the mineral exist including aquamarine, morganite and heliodor, but the green emerald is the most prized and valuable.
A rare combination of uncommon geological and geochemical conditions are required for the formation of emeralds. According to the classical model, Beryllium, essential for crystallization of Beryl, is one of the rarest elements in the Earth’s crust (estimated to be about 2 parts per million), and must be carried up to the surface by pegmatites, which in turn must come in contact with Chromium & Vanadium bearing (ultramafic) rocks to attain the desired colour. Notably, not all pegmatites are Beryllium bearing and even fewer are emplaced within country rocks with adequate Chromium. This, coupled with even more specific temperature, pressure and fluid content requirements, makes emeralds extremely rare and remarkably erratic in its distribution.
Emeralds vary in colour depending on their chromium, vanadium and iron content. In colour they range from bright green with yellowish undertones, to vibrant green with bluish undertones.
Green, naturally. Vivid green, preferably, with lots of depth. Neither too dark nor too light, and evenly distributed throughout the gemstone. Traditionally emerald green should be the perfect balance of blue and yellow – a pure green hue.
The appraisal and appreciation of colour is, of course, subjective to some extent. Emeralds from different origins often have distinctive colours although each deposit can have a range in hues. Zambian emeralds, for instance, often have distinctive secondary hues of blue, which can be utterly mesmerising.
A gemstones clarity has to do with the number of inclusions that it contains, the size of those inclusions and their position within the gemstone. The fewer, smaller and less conspicuous the inclusions, the better.
It is, however, extremely unusual to find a clean emerald. Almost all contain inclusions that are visible to the naked eye. These may be embedded crystals of other minerals, growth lines, cleavages or tiny fractures. They are entirely natural. They are, indeed, a kind of code – one that can reveal the story of the gemstones origin and its subsequent migration from subterranean darkness to the light of day. As such they may be thought to possess their own kind of beauty – some of their patterns are so intricate they are referred to as an emeralds jardin.
Emerald is a relatively hard gemstone – about 7.5 or 8 on the industry-standard Mohs scale, where talc is 1 and diamond is 10. Yet it is also brittle, and this, together with the presence of jardins, can make it a challenging gemstone to cut. The so-called emerald cut – rectangular or square, with bevelled edges – was specially developed to show emeralds off to best advantage while minimising the risk of fracturing or chipping. However, emeralds are also available in cushion, oval and pear cuts. Recently the smooth, dome-shaped cabochon cut has become popular, as have non-traditional slices and rough cuts.
If an emerald is well cut, it will mask colour variation, inclusions and other imperfections, and not create “extinctions” (dark patches). Badly cut gemstones tend to maximise weight at the expense of brilliance.
The gemstone should be well proportioned and symmetrical, with no distortions. It should have sharp facet edges and flat faces – reflections should enter the eye at once, not “roll” across the face – and the gemstone should be free of chips and scratches. When polished well the gemstone should also possess good lustre – in the case of emerald, the desired quality is known as “vitreous”.
With emeralds, beware of vulnerable-looking corners. Because emerald is relatively brittle, it is preferable, from a practical point of view, to choose an emerald cut over other traditional cuts, since it reduces the likelihood of damage to the gemstone.
Emeralds are nearly always oiled to enhance their colour and mask inclusions, and there is nothing untoward about this centuries-old process. The presence of oil may be revealed in a slight iridescence on the surface of the gemstone or one that may be seen inside the gemstone when viewed under a bright light – though iridescence within an emerald can also occur naturally.
Check for concentrations of dye in any cracks in the gemstone – dye is used to improve inconsistent or weak colour.
Emeralds with significant fissures are sometimes filled with a resin or polymer to improve clarity. If so, flashes of orange or blue may be noticed when the gemstone is inspected under a bright light – to an experienced eye, they are fairly easy to spot. It is important to know about fillings, as they can be dislodged during setting or cleaning.
The presence of joining lines along the edges could reveal a doublet emerald. This is when the gemstone has been combined with another material (such as synthetic sapphire, quartz or glass) . If in doubt, put the gemstone in water – you will see a distinct line at the join.
The extent to which an emerald has been treated will affect its value – an emerald with good natural colour and clarity will always be more valuable than one whose qualities have been artificially enhanced. Treatments may also affect the gemstone’s longevity, since emerald is brittle to begin with, and those with fillings may be less durable. So any treatments that a gemstone has undergone must be fully disclosed at the point of sale.
FC Gems emeralds are sold with full disclosure of any treatments to which they may have been subjected. Wherever you buy your emeralds, ask the seller for details.