A laser-drilled diamond is one in which a fine laser channel has been driven from the surface to a dark inclusion, ordinarily a black or dark-brown sulphide or graphite crystal, in order to bleach or remove the inclusion and so improve the apparent clarity of the stone. The technique was commercialised in the late 1960s and early 1970s and is now an entirely standard, fully disclosable diamond treatment recognised by every major laboratory.

The process is straightforward in concept. A pulsed Nd:YAG or similar laser is focused through the polished surface and traces a narrow cylindrical channel down to the target inclusion. Acid (typically sulphuric or hydrofluoric) is then drawn through the channel to dissolve or bleach the dark mineral. The result is an inclusion that reads white rather than black under magnification, and a thin drill hole that is normally visible only with a 10x loupe in transmitted light. In stones treated by the older "internal laser drilling" or KM (Koh-i-Noor Method) variant, the channel does not reach the surface but instead opens an internal feather to the inclusion, leaving a different microscopic signature.

GIA grades laser-drilled diamonds on the regular D-to-Z, IF-to-I3 scale and reports the drilling on the certificate as a clarity-treatment comment, with a separate sketch showing the drill paths. The treatment is considered permanent and stable to all normal jewellery handling, including ultrasonic cleaning, retipping and rhodium plating, although high-temperature operations such as laser welding directly adjacent to the channel can in principle damage the stone. Pricing in the trade reflects the treatment: a laser-drilled SI1 typically transacts at a discount of roughly 15-25 percent against an untreated equivalent of the same colour and weight, depending on how visible the channels are and on overall make.

From an identification standpoint the channels are diagnostic. They are pin-straight, of constant diameter, and almost always terminate at a now-bleached inclusion. They cannot occur naturally and they cannot be confused with growth tubes in other species. A microscope at 30x in dark-field is normally sufficient. Some laboratories also note whether the channel has been infilled with glass, which is a separate and more visible secondary treatment.