Natural emerald tends to be more heavily included than any other kind of gemstone. Most emeralds have not only numerous internal inclusions and fractures, but also tiny surface breaking fissures or cracks. For this reason, the vast majority of emeralds are treated to improve their clarity. This practice is widely accepted in the trade, but there is some controversy surrounding recent developments in emerald treatments.
The traditional treatment for emerald is oiling with cedar oil. Cedar oil is a natural product from cedar trees and is colorless and viscous. It can be produced in a very pure form and has a number of industrial uses. It is used as an immersion oil in light microscopy, as an ingredient in insect repellents, as an anti-bacterial and as a food preservative. It is also often used for its aromatic properties, especially in aromatherapy. Cedar oil has been used for fracture-filling emerald because it has a refractive index that is similar to emerald.
Since cedar oil is so sticky, it is not easy for it to penetrate the microscopic cracks in emerald. So it requires some heat and pressure to do the job. First the emeralds are cleaned, usually in an acid bath. Then the gems are placed in a heated hydraulic cylinder with pure cedar oil and locked tight. The heat liquifies the cedar oil and the pressure helps it penetrate any tiny cracks in the emerald.
After several hours the cylinder is left to cool. The stones are then removed and cleaned. The cedar oil that has penetrated the emerald returns to its thick, viscous state, making it very difficult for it to leak out without the aid of ultrasonic cleaners, excessive heat or harsh solvents.
Traditional oiling is stable but not permanent. Eventually an oiled emerald will require re-oiling to keep it looking at its best. Therefore, a number of attempts have been made to introduce more permanent fillers. These include natural and artificial resins (such as Opticon), polymers and prepolymers. The use of these new fillers has created some controversy, particuarly in cases when the exact filler has not been disclosed.
One important thing that consumers want to know is thedegree to which an emerald has been enhanced. This is perhaps even more important than the type of filler used. Some labs, such as the GIA (Gemological Institute of America) have tried to develop classification schemes that distinguish emerald enhancement as minor, moderate or significant, depending how many surface-reaching fissures have been filled.