Laser drilling is a diamond clarity-improvement treatment in which a pulsed laser is focused through the polished surface of the stone and used to bore a narrow channel down to a dark inclusion. The inclusion, almost always a sulphide or graphite, is then bleached or dissolved through the channel using acid, leaving a white-appearing void in place of the original black spot. The treatment was developed at General Electric and at Israeli cutting houses in the late 1960s and entered mainstream commercial use in the 1970s. It is permanent, fully disclosable, and recognised by GIA and every other major laboratory.

Two principal variants exist. Conventional or "surface-reaching" laser drilling produces a straight cylindrical channel that opens at the polished surface and runs in a near-straight line to the inclusion. KM treatment, sometimes called internal laser drilling, instead uses a series of laser pulses to expand a pre-existing feather until it reaches the inclusion, with no new surface-breaking hole. KM channels appear in the microscope as wormy, irregular planes rather than as straight tubes, and they are often easier to confuse with natural feathers under low magnification.

Identification under a 10x loupe in dark-field is straightforward for surface-reaching drills: the channel is pin-straight, parallel-walled, and ends at a now-bleached inclusion. KM drills require closer examination, because the iridescent feather they exploit is itself a natural feature; the diagnostic clue is that the feather opens into a bleached zone where there should not be one. Both variants can be combined with subsequent fracture-filling, in which case the channel and any reached fissures are infilled with a high-refractive-index glass; that secondary treatment is not permanent and is reported separately.

In trade terms, laser drilling carries a clarity-grade penalty rather than an outright disqualification. GIA grades the stone on its post-treatment apparent clarity but always discloses the drilling. Wholesale pricing for laser-drilled goods typically runs 15-25 percent below comparable untreated material in the SI to I range, with the discount widening for stones in which the channels are visible face-up.