The treatment of the clarity resp. transparency due to the filling of surface-opened fissures improves the appearance of a stone distinctly. Furthermore, the filling with polymers increases the stability.
One of the first descriptions of the use of oil as a filling material for fissures, especially for emeralds, was given by C. Plinius Secundus (55 AD) in his 37th book of natural history. Other historic information are the Papyrus Graecus Holminensis (400 AD), as well as Arabian writings from the 14th century. Since the 1980s artificial resins, which fill the fissures and - depending on the kind of application - could coat the stone too are used to impregnate emeralds. Oil, wax, soft resins and artificial resins are the common filler substances for the clarity treatment of emeralds today.
In the picture above: Colombian Emerald (1.16ct.) before (left) and after (right) the clarity treatment
This DGEmG- and DSEF-Information supplies a comprehensive overview of the current processes and detection methods of this treatment. Here you find the whole article in PDF-format: DGemG- and DSEF- information: Clarity treatment of emeralds