Emerald Clarity Enhancement
Christopher P. Smith
American Gemological Laboratories (AGL)
Chris Smith, President, and owner of American Gemological Laboratories (AGL) educates us on emerald clarity enhancement. In the first part, we are introduced to the basic principles of clarity enhancement. Next, we learn how to judge the extent of clarity enhancement, and finally, Chris provides us with a visual reference to assess the type of filler present.
Emerald is one of those gem materials that is really only enhanced by one method for all practical purposes. That is clarity enhancement. The clarity enhancement process involves the introduction of a filler material into surface reaching fissures. An open fissure containing only air will cause the fissure to be bright, reflective and readily visible because the passage of light is interrupted, being reflected or refracted. However, when a material (in this case a filler) that has a refractive index similar to emerald is introduced into these same fissures, light is able to pass through with much less interruption, effectively reducing their visibility (figure 1). In principle, the closer the refractive index (R.I.) to the gemstone, the less that light passing through the fissure will be refracted or reflected and the better or more efficient the filling material will be at reducing the visibility of the fissures.
Figures 1a and 1b
There are both visible and spectroscopic methods to identify the presence and identity of fillers in emeralds (and other gem materials). In fact, depending on how close of a refractive index match there is between the emerald and filling material, identifying the presence and true extent of a filled fissure may be quite difficult. It is often the presence of air pockets or areas of incomplete filling that are the first evidence that identifies the presence of a filled fissure (figure 2). The bright and reflective nature of air in a fissure makes them far easier to see with a loupe or microscope than a fissure that is thoroughly filled. Common analytical methods available in most gemological labs today also include infrared spectroscopy and/or Raman spectroscopy to identify the specific nature of a filling material.
For the average jeweler, gemologist or appraiser, observing emeralds and other gems under magnification using a standard loupe or gemological microscope remains the most effective method for identifying that a gem has been clarity enhanced. It is common for a filled fissure to exhibit an iridescence or flash effect when examining a clarity enhanced gemstone (Box A). Another simple technique includes looking for traces of where fissures reach the surface while observing the stone in reflected light. In addition, many of the materials used to clarity enhance an emerald will fluoresce to long-wave ultraviolet light (LWUV). As a result observing an emerald in a darkened environment under LWUV can also help highlight fissures that are filled.
Broadly speaking, the materials used to clarity enhance gems may be classified as: Traditional and Modern (Box A). There are those traditional materials which have historically been used to clarity enhance emeralds (and other gems), involving the use of various oils, waxes or natural tree resins. In addition there are modern materials involving various types of petroleum-derived, polymer-type resins which have been increasingly used since the late 1980’s to clarity enhance gems. In today’s market all of these materials are routinely used to clarity enhance emeralds.
In general, using observations alone, without additional techniques such as infrared or Raman spectroscopy, it is not conclusive to identify the traditional or modern nature of a filler. However, there are a couple of good guidelines to recognize some of the modern fillers. These include a distinct yellow and blue flash effect (other colors have also been observed, but these are the most common) and a chalky blue-white LWUV fluorescence reaction. These observations are not seen in the traditional filling materials however not all polymer-type resins display these features.
Determining the extent of clarity enhancement involves three main considerations: the number of fissures, the location of the fissures and the size of the fissures (figure 3). Most gemological laboratories apply a descriptive terminology to describe the extent of clarity enhancement using the terms: None, insignificant, minor, moderate, strong and prominent or an alpha-numeric system (such as F1, F2, F3). With a little practice, every jeweler, gemologist or trader can effectively identify the presence and extent of clarity enhancement in an emerald (or other gem).
In closing, it is important to note that American Gem Society membership (AGS) follow the appropriate guidelines for disclosure. Federal Trade Commission guidelines (FTC) mandate that treatments, such as clarity enhancement, need to be disclosed at the point of sale.
Figure 1: Prior to a clarity enhancement treatment, open fissures in emeralds are bright and reflective. Once a filling material has been introduced into these fissures, light is able to pass through the fissure with less refraction and reflection, make them less visible and improving the apparent clarity of the gemstone. Shown here is an emerald before and after clarity enhancement.
Figure 2: High relief air pockets are commonly the first sign of a filled fissure when looking at emeralds with a loupe or microscope.
Figure 3: LMHC chart for filled fissures combined with AGL bar graph for clarity enhancement
BOX A: American Gemological Laboratories (AGL) is one of only a small number of labs that identifies the nature of a filler present in a gemstone. AGL classifies the various types of filler substances into two broad categories: Traditional and Modern.
Those materials that are classified as traditional include various types of oils (such as Cedarwood and many others), waxes (such as Paraffin) and natural tree resins (such as Canada Balsam). In contrast, those materials classified as modern include the various petroleum derived polymer-type resins, which may be marketed under many different names (such as Opticon, Palm Oil or Palma, Gematrat and ExCel, as well as others).
Oil: In emeralds, oiling is considered the most traditional method of clarity enhancement. Although in the past a vast number of different oils have been used, today the industry generally seems to consider cedarwood oil, to be the preferred choice. Cedarwood oil generally displays a “rainbow iridescence” when viewed with magnification (figure A1). Cedarwood also tends to have a weak yellow reaction upon exposure to long-wave ultraviolet light.
Wax: It is not really known when the trade started utilizing a wax for the purpose of clarity enhancing emeralds. However it seems to make sense in that wax becomes liquid at a very low temperature allowing it to enter fissures readily and then naturally becomes less viscous (i.e. somewhat solid) at room temperature. It has a refractive index of approximately 1.43. Similar to Canada Balsam, wax can have a slightly “gritty” appearance when observed in emerald fissures with magnification (figure A2) and it displays a yellow reaction when exposed to long-wave ultraviolet light.
Tree Resin: Although there are numerous resins that could be used for this purpose, the resin known as Canada Balsam seems to have been used most frequently to fill emeralds. Canada Balsam is a viscous, material derived from the balsam fir tree. Over time the essential oil in Canada balsam evaporates, leaving behind the resin as a transparent varnish-like material. As it solidifies it can take on a slightly yellowish, “gritty” or dendritic appearance in the microscope (figure A3). It also tends to exhibit a strong yellow reaction to long-wave ultraviolet light.
Polymer-Type Resin (also commonly referred to as Epoxy Resin): Today, a broad range of petroleum-derived resins are used to fill fissures in emeralds, or more commonly they are combined with a polymerizing agent to harden the resin, creating an epoxy or polymer. Some of the more common polymers used to enhance emeralds are referred to as “Opticon”, “Permasafe”, “Palm Oil”, “Gematrat” and “ExCel”. Fissures filled with such fillers commonly exhibit the classical orange and blue “flash effect” when viewed with magnification (figures A4 and A5). Polymer-type resins tend to appear bluish-white to chalky yellow when observed under long-wave ultraviolet light.