A drill hole is a narrow, tubular channel bored into a polished diamond by means of a focused laser beam, created for the purpose of reaching an otherwise inaccessible dark inclusion and rendering it less visible. The technique is one of the most widely practised clarity-enhancement procedures applied to diamonds, and its results are considered a permanent treatment by every major gemmological laboratory. Because the channel itself constitutes an additional clarity feature — one that did not exist in the rough — drill holes must be disclosed at every point of sale, and their presence is noted explicitly on laboratory reports issued by the GIA, IGI, AGS, and other recognised bodies.

The Laser Drilling Process

Modern laser drilling employs an infrared Nd:YAG (neodymium-doped yttrium aluminium garnet) laser, typically operating at a wavelength of 1,064 nanometres, to vaporise a microscopic column of diamond crystal from the surface to the target inclusion. The beam is precisely focused and delivered in pulses, ablating material in a controlled path that measures roughly 0.2 to 0.5 millimetres (200–500 microns) in diameter — narrow enough to be difficult to detect with the naked eye, yet sufficient to create a channel through which bleaching agents can be introduced.

Once the channel reaches the dark inclusion — most commonly a graphitised carbon crystal or a heavily included fracture that has darkened through natural processes — the diamond is immersed in a strong oxidising acid, typically concentrated sulphuric or hydrochloric acid, which is drawn into the channel by capillary action or applied under pressure. The acid bleaches or dissolves the dark material, converting an opaque black or dark brown inclusion into one that is colourless or near-colourless, and therefore far less conspicuous against the diamond's body. The channel itself remains as a permanent, fine tube extending from the surface to the former inclusion site.

Morphology and Detection

Under magnification, a conventional laser drill hole presents a characteristic appearance that experienced gemmologists learn to recognise reliably. The entry point at the surface is typically a small, slightly raised or depressed pit with a circular or slightly oval cross-section. The channel walls are smooth and glassy, a consequence of the laser's intense heat re-melting the surrounding diamond material as it ablates. The tube itself is generally straight, though the operator may angle the beam to approach an inclusion from the most advantageous direction. At the far end of the channel, the bleached inclusion site may appear whitish, cloudy, or entirely transparent, depending on how completely the dark material was removed.

Detection is most reliably performed under darkfield illumination with a loupe or gemological microscope at 10× to 30× magnification. The channel catches light differently from natural feathers or cleavages: it appears as a bright, reflective tube when the stone is rotated, and the circular entry point is a consistent indicator. Fibre-optic or oblique illumination can make the channel glow distinctly. Experienced gemmologists also note that the bleached inclusion site often retains a slightly frosted or etched texture rather than the clean, transparent appearance of a natural void.

KM (Kiduah Meyuhad) Drilling and the Yehuda Fill

A variant technique, developed in Israel and sometimes referred to by the Hebrew abbreviation KM (for kiduah meyuhad, meaning "special drilling"), creates a broader, less regular channel that follows the natural plane of an existing fracture or feather within the stone, rather than boring a straight tube. This approach exploits pre-existing internal planes, producing a channel with an irregular, worm-like or branching morphology that can be more difficult to identify as a treatment because it superficially resembles a natural feather. GIA researchers documented this technique in detail in Gems & Gemology during the 1990s, and it remains a subject of careful scrutiny in laboratory examination.

Both conventional drill holes and KM channels may be left open or subsequently filled with a glass-like substance — most famously the proprietary material associated with the Yehuda process, introduced by Zvi Yehuda in the 1980s. This filling, a lead-rich glass with a refractive index close to that of diamond (approximately 2.42), is introduced under heat and pressure and serves to mask the channel optically, making it nearly invisible under casual inspection. Filled drill holes present a characteristic "flash effect" under magnification: as the stone is tilted, the filling flashes between an orange-pink and a blue-green colour, a diagnostic feature caused by the difference in refractive index between the glass and the surrounding diamond. This flash effect is the primary detection tool for filled channels and is described in GIA's published research on clarity enhancement.

Laboratory Grading and Disclosure

The GIA does not assign a standard clarity grade to laser-drilled diamonds on its grading reports; instead, it notes the presence of laser drilling in the comments section and describes the treatment on the report's face. This policy reflects the laboratory's position that laser drilling is a permanent alteration of the stone's natural condition, and that consumers are entitled to full disclosure. Other laboratories, including the IGI and HRD Antwerp, follow broadly similar disclosure protocols, though the precise language and placement of the notation varies.

Filled drill holes occupy a more complex position. Because the Yehuda-type glass filling is not permanent — it can be damaged by jeweller's torches, ultrasonic cleaners, and certain acids used in routine jewellery repair — the GIA declined for many years to grade filled diamonds on standard reports, issuing instead a separate "clarity-enhanced diamond" identification report. The impermanence of the fill distinguishes it fundamentally from the drill hole itself, which, being a void in diamond crystal, cannot be reversed or removed.

Market Context and Valuation

Laser-drilled diamonds trade at a meaningful discount to untreated diamonds of equivalent apparent clarity. The precise discount varies with market conditions, the degree of improvement achieved, and whether the channel has been filled, but buyers and appraisers consistently apply a reduction to reflect both the treatment itself and the reduced liquidity of enhanced goods in the secondary market. A diamond that appears to be SI1 in clarity after laser drilling and bleaching will command substantially less than a natural SI1 stone, because the treatment is permanent, disclosed on the report, and cannot be reversed to restore the stone to its pre-treatment condition.

In the trade, laser-drilled diamonds are sometimes marketed as "clarity-enhanced" without further specification, a practice that has attracted regulatory attention in several jurisdictions. The United States Federal Trade Commission's guidelines on jewellery marketing require disclosure of all treatments that affect value, and laser drilling unambiguously falls within that category. Reputable dealers disclose the treatment in writing at the point of sale and ensure that any accompanying laboratory report reflects the enhancement.

For collectors and investors focused on untreated diamonds, the presence of a drill hole — even one that has been expertly executed and is difficult to detect without magnification — is generally considered a disqualifying feature for premium pricing. High-value transactions in the investment-grade diamond market routinely specify "no clarity enhancements" as a condition of purchase, and major auction houses including Christie's and Sotheby's note the absence of treatments as a positive attribute in catalogue descriptions of significant stones.

Summary of Key Characteristics

• Channel diameter: typically 200–500 microns (0.2–0.5 mm)
• Created by: infrared Nd:YAG laser at 1,064 nm wavelength
• Purpose: access dark inclusions for acid bleaching
• Permanence: the channel is permanent and irreversible
• Detection: darkfield microscopy, 10×–30×; circular surface pit; reflective tube walls
• Filled variant: Yehuda-type lead glass; identified by orange/blue flash effect
• Laboratory treatment: noted in comments on GIA and other major reports
• Market effect: discount to untreated goods of equivalent apparent clarity

Further Reading

• GIA Gems & Gemology — Laser Drilling of Diamonds (Shigley et al.)
• GIA Gem Encyclopedia: Diamond
• GIA Gems & Gemology — Fracture Filling of Diamonds