Certain professionals treat diamonds in an attempt to improve their quality. Two methods exist to improve clarity: glass filling and laser drilling.
Glass filling is the vacuum filling of fractures with leaded glass, bismuth, and bromine. Cracks can be naturally present in the rough diamond or they can happen during diamond processing. Such cracks have a negative impact on the clarity, and hence the price, of the stone. The objective of glass filling is to make these cracks and cleavages less visible. Specially treated glass with a refractive index very close to diamond is used to fill in the cracks. This process allows professionals to treat inclusions that touch the surface of the diamond. These are cavities within the stone which can put the durability of the diamond at risk especially if they open the surface of the stone or if they touch one another. This treatment can actually cause cracking and yellowing of diamonds.
Firms that practice this treatment method include Zvi Yehuda in Israel, Koss, and Goldman-Oved. The treatment can also be applied to rubies. One can detect the treatment in the little white lines on the surface and a blue or pink “flash effect” due to the slightly different refractive index. Tiny air bubbles may also be present after glass filling. These bubbles never appear naturally, so their presence is a sign of treatment. A binocular microscope with dash fields is needed to see such indications. Certifying laboratories refuse to grade glass-filled stones.
The second treatment is laser drilling. Laser drilling allows the professional to reach black inclusions deep in the stone which cannot be removed by cutting or polishing. First the professional digs with the laser through the table or the bezel, damaging the diamond in order to create an entry point. S/he then cleans the black inclusion with hydrochloric acid. The void is then filled in with doped glass. Firms practicing this treatment engrave “KM” on the girdle of the stone. “KM” stands for “Kiduah Mayuhad,” which in Hebrew means “special boring.” This practice was first tried in 1963.
It is possible to detect this type of treatment because of the channel created to reach the inclusion. Usually this channel is shaped like a millipede. Diamonds that have undergone this type of treatment can receive color and clarity gradings from certifying laboratories, but the laser drilling must be indicated on the certificate.