Clarity refers to purity of a diamond, whether colorless or colored, a clear gem appears uniformly brilliant and exhibits luster from all angles and facets. Clarity strongly evaluates a diamond's worth. Fractures, generally called feathers due to feathery white appearance, are the most common inclusions in diamond. Other common inclusions are solid crystals of other materials like garnet. The size, color and position of inclusions can reduce the diamond's value even if it is good in other gemological factors. In such a case, treating and enhancements can be helpful in increasing the value of a diamond.
Diamond is combustible and therefore laser drilling techniques selectively target and substantially reduce the visibility of fractured part of the crystal and iron oxide stains within the fractures. Laser drilling of diamonds started around mid 1980's and I soften followed by glass infilling. This process requires use of an infrared laser( 1060 nm wavelength ) to bore extremely fine holes ( less than 0.2 mm) to approach the inclusion. To initiate the combustion by laser, a layer of amorphous carbon or other energy absorbent material is applied to the diamond's surface before laser drilling can begin, as diamond is transparent to the laser's wavelength.
The laser drills a small tubular passage to the inclusion and the diamond is then immersed in sulphuric acid to dissolve iron oxide stains in the crystal. This method is unfit for removing impurities which are diamonds themselves, because diamond cannot react with sulphuric acid. These bore holes are readily detected under a microscope, and can be almost straight, but sometimes can be slightly bent having a "wrinkled" appearance. In reflected light, these holes can be seen as dark circles gapping the facets. The diamond material destroyed during the drilling process thus replaced by glass using fracture filling techniques.
Due to the visibility of the drilled holes in the diamond, research began for devising a method to fill up this visible flaw. Glass filling is the common method used for filling up the fractures. The glass used in filling these fractures is specially formulated having refractive index approximating to that of diamond. This method was pioneered by Zvi Yehuda of Ramat Gan in Israel, and Yahuda is now a brand name applied to diamonds treated with this process and has changes little since its origin. Another attempt in glass filling method was made by Koss and Schechter, an Israel based firm. They used a halogen based glass but it proved unsuccessful. The details of Yehuda's process have been kept secret, but the glass used for the method is believed to be lead ox chloride glass. New York based firm, Dailies, also uses lead bismuth ox chloride glass for fracture filling.
Though the glass is nearly having the same refractive index to that of diamond, but is can still be detected by a trained gemologist under a microscope. The visible features include surface reaching bore holes and fractures in drilled diamond, air bubbles and flow lines within the glass.
Another feature that represents a treated diamond is flash effect, when bright flashes of color are seen when the treated diamond is rotated, ranging from blue to purple to an orange or yellow, depending upon the lighting conditions. The flashes are most intense when the plane of view is parallel to the fracture plane. The flash effect can be diminished in strongly colored diamonds. For example the blue colored diamonds would not let the blue flashes to be clearly seen, whereas the yellow and red parts will be more visible. The color of the filling glass is also different from that of the treated diamond. It may be yellowish to brownish which can impact the color grade of the diamond after fracture filling and a diamond may go down a color grade. Therefore, fracture filling is performed on large diamonds. But diamonds having carat weight of 0.02 carats have been fracture filled.
The fracture filling of a diamond is not desirable in industry as well as public, as the glass has a very low melting point and therefore melts out of the diamond under the heat of a jeweler's torch! If the jeweler is unaware of the treatment, the heat of torch can lead to degradation of clarity, even shattering. The treated diamond if placed in an ultrasonic cleaner may also not survive. Yehuda diamonds are therefore warrantied for retreatment for any debasement of the glass filling.
GIA, and other major gemological labs do not issue certificates for fracture-filled diamonds. Some labs which do certify these diamonds provide the certificates regarding the original properties of the diamond, before the treatment.