Emerald Quality Factors
As with other coloured stones, it requires a well-trained eye to recognise the sometimes subtle variations that make significant differences in emerald value. This is especially true in the higher qualities.
The most desirable emerald colours are bluish green to pure green, with vivid colour saturation and tone that’s not too dark. The most-prized emeralds are highly transparent. Their colour is evenly distributed, with no eye-visible colour zoning. If the hue is too yellowish or too bluish, the stone is not emerald, but a different variety of beryl, and its value drops accordingly.
The intensity of the green in the finest emeralds might not be equalled by anything else in nature.
Chromium, vanadium and iron are the trace elements that cause emerald’s colour. The presence or absence of each and their relative amounts determine the exact colour of an emerald crystal.
Emerald appearance is sometimes associated with its mine location. Colombian emeralds are said to have a warmer and more intense pure green colour. Zambian emeralds are said to have a cooler, more bluish green colour. In spite of these theories, the truth is that emerald appearance overlaps between sources.
Emeralds typically contain inclusions that are visible to the unaided eye. Because of this, trade members and some consumers understand and accept the presence of inclusions in emeralds. Eye-clean emeralds are especially valuable because they’re so rare.
Emerald inclusions are often described as looking mossy or garden-like. They’re sometimes called “jardin”, which is French for garden.
In coloured stones, transparency and clarity are closely linked. This is especially true for emeralds. The trade generally accepts eye-visible inclusions in higher-quality emeralds. But when the inclusions have a negative effect on transparency and clarity, they also dramatically reduce value.
In early 2000, the GIA Laboratory began offering a classification service for emerald clarity treatments. The laboratory examines a loose stone and provides an Emerald Report that includes a digitally generated colour image of the stone taken at the time it’s examined. The report describes the level of clarity enhancement as minor, moderate or significant. The GIA Laboratory emphasises that it uses the classification system only to evaluate the level of treatment, and not to offer an overall clarity grade for the stone.
The cutter must consider the rough’s depth of colour, durability and inclusions when making cutting decisions. Mistakes cause weight loss, which greatly reduces the value of a potentially valuable gem.
Four characteristics of emerald crystals make them difficult to cut. First, almost all emeralds have significant fractures (sometimes called fissures in the trade). A cutter must design the cut to minimise the effect of those fractures on the finished stone.
The second factor is due partly to those inherent fractures: Emeralds are more brittle than a gem like corundum. This makes them vulnerable to damage during cutting, polishing and setting, or even during careless daily wear. The emerald cut can help protect against damage because the vulnerable corners are faceted and provide a comparatively safe place for prongs.
Third, because colour is so important in establishing an emerald’s value, the cut must maximise the effect of hue, tone and saturation. The cutter can affect colour by adjusting an emerald’s proportions and number of facets. The cutter can darken a pale stone with a deep cut, a small table and fewer facets, or lighten a dark stone with a shallow cut, a large table and additional facets.
Fourth, the bluish green to yellowish green dichroism of many emerald crystals encourages the cutter to orient the table so it’s perpendicular to the crystal’s length. That way, the more apparent colour in the cut gem is the bluish green that so many emerald lovers prize.
Colombian rough is especially challenging due to the distribution of colouring agents during formation. Its colour is more intense closer to the surface. Without careful planning and cutting, the finished stone might be much lighter in colour than the original material.
Fashioned emeralds come in a wide range of sizes. There are emeralds in museums and private collections that weigh hundreds of carats. At the other extreme are tiny emeralds that weigh fractions of a carat.
The Sandawana emerald mine in Zimbabwe is known for its tiny, yet vividly coloured stones. Its emeralds are as small as 1 mm square, but they’re still intensely green. The mine’s cut stones average about 0.05 to 0.25 carat, and rarely weigh more than 1.50 carats. Popular jewellery sizes are usually somewhere in between.
The smallest sizes range from 1 mm to 5 mm, with weights from 0.02 to 0.50 carat, while 1 to 5 carat stones are popular as centre stones. Prestigious pieces of jewellery can include emeralds that weigh over 20 carats.
Quality-for-quality, the price of emerald can rise dramatically as the size increases.