The methodical name for the treatment behind clarity enhancement is fracture filling.
Fracture filling is the main treatment behind the enhancement process. This treatment is aimed at eliminating inclusions called feathers.
A feather is a very small internal fracture in the diamond. Since the vast majority of rough diamond material is not flawless, these fractures usually occur naturally. There are some cases though where these fractures can be even produced by accident while the diamond is being manufactured.
A feather is visible because the internal characteristics of the diamond are flawed. As light passes in between the two walls of the fracture and it is reflected outside. Thus, the fracture is visible and appears as a minute white crack.
During the fracture filling treatment, a microscopic amount of a distinct material is boiled into the two walls of the feather. This material possesses a very fascinating quality. It changes the light diffraction factor of the crack. This makes the light pass through the walls of the feather, instead of being reflected from them. Thus making the feather invisible. The fracture still remains in the diamond, but instead of being visible, it is now invisible. The treatment can be removed by using strong industrial acids or extreme heat of over 300 degrees Celsius (572 degrees Fahrenheit). In any case, if the material has been removed, the diamond can be easily treated once again.