Chromium, the very element that gives emeralds their beautiful colour, also causes a brittleness that results in internal fissures that affect the clarity and this is further intensified by the fact that the emerald cut (the most common emerald shape) exacerbates the effects of this phenomenon. Emeralds appear clearer when these fissures are filled (where they meet the surface) with oils, polymers and resins. The clarity enhancement achieved by this is normally assessed by certification authorities as: "minor, moderate or significant." The level of clarity enhancement, or oiling, can have a profound effect upon the value of the stone and it's appearance (see the before and after comparison below).
Clarity enhancement of emeralds is by no means a new phenomena. Historical accounts of this practice have been traced back as far as A.D. 77. In Colombia Cedar oil has become the staple material used for the oiling process but it is interesting to note that, unlike many other coloured gems, it is impossible to alter the colour of an emerald. Every emerald is natural in colour but it's clarity can be influenced. A small degree of oiling is commonplace with emeralds, it is the degree to which this has taken place that is key to its effect on price. As a general rule, the more surface reaching fissures that are present, the higher the degree of oiling. It is possible to identify where surface reaching fissures have been oiled, under magnification and observed in reflected light.
(Fissures in an emerald under a microscope)
The process of clarity enhancement can only be effective where fissures reach the surface of the stone. Consequently, very heavily included stones cannot be enhanced significantly because their imperfections are largely within the stone itself and do not reach the surface. By the same token, a stone that looks very clean can have one long fissure opening, enabling very significant enhancement to take place. Before purchasing an emerald you want to know the degree of clarity enhancement: "slight, moderate or significant." This can be defined as the difference between the probable appearance of the stone if were un-oiled, versus it's appearance when oiled. If you want to purchase an emerald with "no enhancement," the price premium can be as high as 50% but most professionals agree that the difference between these stones and those that are "slightly" enhanced is almost imperceptible to the human eye! Furthermore, even the most revered certification laboratories have been known to confuse "no" oil with "slight" oil.
(A stone being treated in Colombia)
Stones that are purchased within the category of "moderate" enhancement form the majority of those on the market. As stated by the World-renowned gemmologist and emerald expert Ronald Ringsud: "emeralds with 'heavy' enhancement are generally to be avoided, since the fissures in the stone may extend to the edges or corners, where the stone is most subject to stress from mounting in jewellery." They are however the cheapest emeralds available and can represent very good value for money! When purchased for items of jewellery that do not suffer from heavy wear and tear, such as earrings, they can still make very attractive pieces. Enhancement with oils or resins does not increase the weight of an emerald, so a 0.56 carat heavily oiled stone would still have measured in at 0.56 carats prior to treatment.
(A simplified explanation of the oiling process).
There has been much discussion over the permanency of enhancement techniques. The most authoritative study that has been carried out in this area to date was published in 2006 by the GIA researcher Mary Johnson. She stated that after extensive tests two thirds of treated stones did not noticeably change appearance over time, even over extended periods of time, and that the remaining third were often degraded in large part due to the use of solvent cleaners. She also asserted that even when the appearance of a stone did deteriorate, it could simply be re-oiled and brought back to its former lustre. The avoidance of stones with more than one or two fissures in the crown has often been cited as the best way to avoid the loss of enhancement oil over time and the best results can often be achieve with light cedar oil, without the need to resort to heavier resins and polymers (Gems & Gemmology, 2006)
(Cedar oil is the simplest and most effective method of enhancing an emerald)
Although extremely rare, some very crude methods of enhancing the colour of an emerald have been attempted, such as painting the stone and the use of dyed epoxy resins, but these are so easily detectable that they are very seldom used. These methods have also been used to create so called "composite" emeralds and of course most industry experts agree that these should be avoided at all costs! It is however possible for the colouration of an emerald to be slightly altered by the refraction of light within a fissure or fissures inside the stone itself and this is an entirely natural phenomenon. This refracted discolouration of light within the emerald can also occur as a result of the reflective properties of the resins inserted into the surface fissures of the stone. This most commonly manifests itself as a blue or orange 'flash' along the line of the fissure within the emerald. This has resulted in the phenomenon being known as the "flash effect." It is interesting to note that this effect does not occur when cedar oil is used as the enhancing agent.
(A Colombian emerald displaying it's beautiful refractive qualities)
In conclusion, there are many misconceptions regarding the process and effects of "oiling" or "clarity enhancement" within the emerald and coloured stone market. Whilst "no oil" stones will always attract a hefty market premium, it is important not to overlook the wonderful array of stones within the other three categories. The methods of oiling stones vary tremendously but Cedar oil has by far the best effect. Stones with more than two surface fissures in the crown are the ones to be avoided and certification, whilst important, is not the only indication of the true provenance of an emerald.