Laser drilling is a treatment used to reduce the visibility of dark natural inclusions that reduce a diamond’s clarity grade. It involves drilling with a laser beam a minute tunnel through the diamond, in order to remove a dark spot buried inside the stone.

To the untrained eye the laser tunnels appear to be natural inclusions, and they too may be fracture filled to further improve clarity

Typically, a laser beam with a diameter less than 0.02 millimeter is used. The concentration of light on an extremely small  area of the diamond causes its temperature to rise to a point at which it evaporates, forming the tunnel thatch stretches from the surface to  the inclusion. The inclusion itself is then is then vaporized with the laser, and the entire area is often bleached to conceal any residual dark material.

Laser drilling is easily be detected under a loupe with 10X magnification. If it is fracture filled,  the filler material in the drill tunnels will  show color flashes.

In 2000, the Gemological Institute of America (GIA) reported that it has come across diamonds that had been subject to a new type of laser drilling treatment, which does not have  the surface-reaching drill hole that traditionally had been associated with the treatment.

According to GIA, with the new treatment, one of more or more pulsed lasers are focused on a near-surface inclusion, causing it to heat and expands, thereby creating sufficient stress to extend an existing cleavage to the surface. Once that has occurred, boiling the stone in acids will reduce the visibility of dark inclusions.

For detection purposes the alternative method of lasering may leave evidence of one or more internal channels within or adjacent to the cleavage or adjacent to the inclusion, but there are no surface-reaching drill hole. At first glance, the treated area could appear to be a natural feather.

Narrow tunnels drilled by laser that travel from the surface of diamonds to what previously were dark inclusions within the stone.

Worm-like laser tunnels, drilled through the stone to help remove black inclusions, can appear to the untrained eye as a natural imperfection