The inclusions in emeralds are often extremely fine gas pockets or fissures. The difference in refractive index or light bending between the emerald material and the air in the fissure makes the fissure more visible. To disguise the inclusions, numerous materials have been used to fill them. If the refractive index of the filler is similar to that of emerald then it makes the inclusion less visible.
Considerable discussion has ensued over the different materials used for this purpose, be they oils or resins or waxes, either natural or man-made. The Gemological Institute of America has suggested that the terms natural and synthetic not be used in this regard. They suggest that presumed natural be used for all substances that are naturally occurring, because in many cases these essential oils, other oils, resins and waxes cannot be distinguished from the same materials made in a laboratory. At the same time the GIA suggests that the synthetic be avoided altogether, because it has a very different meaning in chemistry than in gemology. The GIA suggests using the term artificial resins for all pre-polymers and polymers that have no natural equivalents. The names of some artificial resins that you might find reference to as emerald fillers are Opticon 224, Araldite 6010, and others.
There are several issues involved here, including the durability of the material that is being used as filler, discoloration, specific visual effects caused by the filler, and traditional or customary use. From a legal point of view, full disclosure of any treatment or enhancement that is known to have been used or detectable by technical means may be sufficient. However, in the Colombian emerald trade there is a clear distinction between the use of presumed natural essential oils, particularly cedar oil, and the artificial resins. Cedarwood oil is the traditional clarity enhancer that has been used on Colombian emeralds for generations. Its use is very wide spread, not to say ubiquitous, in the Colombian emerald trade. Cedar oil provides clarity enhancement while at the same time not hiding the fissures completely, and thus allows the buyer to detect their presence. Also, cedarwood oil is viscous and tends to remain in the stone; and in the thin films that are formed inside the fissures it is essentially colorless.
If you wish to be completely sure that an emerald you buy has received no clarity enhancement you need to look carefully at the stone with a loupe. If the stone is perfectly clear, that is to say that the material is compact and perfectly formed with no inclusions, there are no fissures within it to accept any type of filler. Also, if fissures are present but they do not reach any of the stone surfaces, then likewise there is no way for a filler to find its way into the stone. The policy at Mister Emerald is to accept cedar oil as a clarity enhancer, and to reject all other enhancement methods. We have chosen this course because we do not believe that any of the artificial resins will permanently maintain their transparency and integrity, and if deteriorated they are very difficult if not impossible to remove.