In light or colorless gemstones, dark inclusions are more easily seen than colorless ones, so a means of lightening dark inclusions was developed using lasers. In laser drilling treatments, a narrow channel is laser drilled from the stone’s surface to the dark inclusion. Once the open channel has been created, a mixture of boiling acid is introduced under pressure and the inclusion is subsequently bleached or dissolved. Care is taken so that the drill hole is not obvious and in most cases, it is only visible with magnification from certain angles. The drill channel is less than one-tenth of a millimeter in diameter, and when observed from the right angle, appears as a fine, straight hair. This process can be done several times to the same stone.
The ‘KM treatment' is a newer technique in the laser drilling process. ‘KM' is the abbreviation of the Hebrew term, Kiduah Mayuhad, meaning ‘special boring'. The process involves burning a passage from the inclusion to the surface of the stone - the passage looks more like a natural inclusion than a drilled channel, and is more difficult to identify.
Laser drilling and bleaching is a permanent and stable process that results in an improvement in the apparent clarity of a diamond.