In the gemworld, nothing compares to the soft green of an emerald. Owing to its rich green hue primarily to chromium, this variety of the mineral beryl is usually found in hydrothermally altered schists and pegmatites.
The most significant producer of fine quality emeralds remains Colombia, against which all other emeralds compare. Colombian emeralds contain elevated amounts of vanadium adding vibrancy to their colour. They also distinctly lack iron which subdues any fluorescence. Some stones may even feature red fluorescence. Muzo, a source fabled since the times of the conquistadores for its ‘old mine’ stones – an often misused term signifying specimens of established provenance and superior quality – still produces rich green emeralds with a slightly yellowish secondary hue. Chivor, another significant mine stands out for its slightly more bluish green emeralds.
The views on colour are divided amongst emerald experts with some of the greatest connoisseurs preferring a warm very slightly yellow secondary hue that frames the Green, whereas others along with the market at large seem to prefer the cooler touch of blue which infuses richness. The latter hue is particularly emphasized by artificial light, whereas natural sunlight will highlight any yellow. Arguably the best stones perhaps feature both pleochroic colours balancing each other out. Generally, bluish green is aligned with the crystal-axis whereas yellowish Green is exhibited perpendicular to the c-axis. This is of great relevance to the cutter during the planning process. In terms of tone medium to medium dark stones are clearly preferred over those that are either too light or ‘over-colour’.
Emeralds are very rarely clean and minor inlusions, even at the top end, are readily accepted. Inspite of this, stones should, however, feature a fine, crisp and non-turbid crystal.
Besides common three-phase inclusions, Colombian emeralds rarely feature the highly prized ‘Goita de Aceite’, literally ‘Oil drop’ or ‘butterfly’ effect. Hexagonal inhomogeneities born through multiple episodes of chemical etching and regrowths during emerald genesis are responsible for this phenomenon. Usually aligned as bands, this feature causes fine blurring and a slightly oily appearance. This mellowing effect – similar to silk in sapphires – leads to light refraction, improving the visual appearance of a stone.
Since emeralds are rarely clean, oiling and worse various colourless resins are often applied. Even dye finds its way into lower qualities. By reducing refraction along fissure boundaries the fillers reduce the visual impact of any cracks.
Unsurprisingly considerable premiums anywhere from 20 to 50% and more are nowadays paid for unoiled over ‘minor’ oiled fine quality emeralds. Higher prices for unoiled gems may also be explained by the scarcity of suitable material devoid of any microfissures. This is largely due to increased mechanization and widespread use of dynamite in mining.
The preeminent gemological laboratories categorize clarity enhancement as insignificant, minor, moderate, significant and excessive. Anything up to and including minor (SSEF) oil only – the benchmark grade – is accepted by the trade in fine, investment grade stones, also given that many other quality factors in a stone outweigh oiling as the sole criterion. Moderately oiled stones designated to fine commercial jewellery, however, can represent good value for money. Such emeralds are very commonly seen in the trade and generally well received. Only colourless synthetic oils or cedarwood oil are acceptable. Unlike hardened resins which must be avoided, these oils do not alter or change colour under UV exposure and are easily removable.
Besides Colombia, Brazil, (Itabira, Nova Era) remains arguably the largest producer. Although its best material is similar to that from Colombia, lower qualities tend to be darker and may contain characteristic black mica. Afghanistan (Panshir) and Pakistan (Swat Valley) have been historically significant producers of some very fine emeralds, often adorning Mughal pieces. These emeralds can be very vibrant and pure green. Zambia is a more recent producer and stones which are not overly blue are in good demand. Due to their high iron content they may sometimes appear darker. The latest exciting origin is Ethiopia which has produced some utterly stunning stones rivalling the best Columbian or Afghan emeralds by hue. Lesser sources are the Ural mountains of Russia producing lighter material and Sandawana in Zimbabwe whose intensely coloured small-sized yellowish Green material lends itself well to cutting spectacular small stones. Ethiopia is the latest source of very fine coloured goods similar to Afghan material. Top pieces from this new source albeit rare, can be absolutely mesmerizing.
The most preferred cut in emerald remains the classic emerald step cut which was conceived to maximize yield from the natural rough elongate hexagonal prisms. Cushions are also desirable as are pear shapes.