Introduction
Some diamonds contain dark inclusions — crystals of graphite, pyrrhotite, or other minerals trapped during formation — that sit beneath the surface, beyond the reach of any cutting angle. Laser drilling changed a cutter's options.
By directing a focused laser beam through the diamond to reach a dark inclusion, gemologists create a microscopic channel — a pathway for acid or heat to bleach or dissolve the offending material. The inclusion becomes less visible. The clarity improves. And unlike fracture filling, the result is permanent. The drill channel, the bleached inclusion, and any residual void remain part of the diamond for its lifetime.
A laser-drilled diamond can be cleaned, repaired, resized, and worn without concern that the treatment will degrade. But the drill hole is there — visible under magnification, recorded on the grading report, and factored into the clarity grade.
How Laser Drilling Works
The Principle
Dark inclusions are visible because they absorb light the surrounding crystal transmits. A laser solves the access problem: the dark inclusion absorbs infrared laser energy far more readily than the transparent diamond crystal, allowing the beam to vaporise a path through the stone to the target.
The Process
The diamond is mounted under a focused infrared laser — typically Nd:YAG at 1064 nm. The beam fires in controlled pulses, burning a narrow channel through the crystal until it reaches the inclusion.
Once open, the diamond is immersed in a strong acid bath — usually boiling hydrofluoric and sulphuric acids. The acid travels down the channel by capillary action and dissolves or bleaches the dark inclusion. In some cases, heat alone is sufficient to alter the inclusion through oxidation.
Where a conspicuous dark crystal once sat, a less visible white or transparent remnant remains. The drill channel — typically 20 to 50 micrometres in diameter, thinner than a human hair — stays open or fills with residual material during normal wear.
Two Types of Laser Drilling
Traditional Laser Drilling
Traditional drilling produces a straight, cylindrical channel running from the diamond's surface directly to the inclusion. Under 10× magnification, these channels appear as fine, perfectly straight tubes — a geometry that does not occur in nature. Their linearity is diagnostic: natural inclusions and fractures follow crystallographic planes and stress patterns, never ruler-straight lines.
Traditional drill holes are the easiest form of laser drilling to detect. They are visible in darkfield illumination as bright, reflective tubes, and they cast characteristic shadows in transmitted light. GIA plots them on the clarity diagram as a distinct symbol and adds the report comment "Laser drilled."
KM (Internal) Laser Drilling
The KM method — named after the Koss and Moolenaar technique — takes a different approach. Instead of drilling a straight channel, the laser heats the dark inclusion, causing it to expand. The expansion fractures the surrounding diamond along existing planes of weakness — feathers or cleavage directions. The resulting micro-fracture network connects the inclusion to the surface, providing a pathway for acid treatment without any straight drill channel.
Under magnification, KM drilling produces irregular, feather-like fractures radiating from the inclusion site. These resemble natural feathers, making detection more challenging than with traditional drilling.
However, KM-drilled diamonds are not undetectable. Trained gemologists identify them by the association pattern: a bleached inclusion surrounded by feathers that radiate outward from the treatment site and connect to the surface in ways that correlate with the inclusion's position rather than natural formation stress. GIA identifies these diamonds with the report comment "Internally laser drilled" and plots the features on the clarity diagram.
Permanence
Laser drilling is a permanent, irreversible modification. The drill channel or stress-induced fractures are physical alterations to the diamond's crystal structure — no cleaning method, temperature, or chemical will undo them. This distinguishes laser drilling from fracture filling, which degrades over time.
For the owner, permanence is practical. A laser-drilled diamond requires no special care beyond what any diamond demands. It can be cleaned ultrasonically, steamed, repaired, and worn daily without concern. The diamond you buy is the diamond you keep.
GIA Grading of Laser-Drilled Diamonds
Unlike fracture-filled diamonds — which GIA refuses to grade on a standard report — laser-drilled diamonds do receive full GIA Diamond Grading Reports. The permanence of the treatment allows GIA to assign a meaningful clarity grade, because that grade will not change over time.
However, the treatment is always noted:
- Drill holes are plotted on the clarity diagram using a specific symbol.
- The comments section states "Laser drilled" or "Internally laser drilled."
- The clarity grade reflects the diamond's treated state — it accounts for both the improved appearance and the remaining evidence of treatment (the drill channel, residual inclusion material, and any associated fractures).
A laser-drilled diamond graded VS2 means it meets VS2 criteria in its current, treated condition. The clarity before treatment would have been lower — often significantly so.
Detection Under Magnification
At 10× magnification — the standard for diamond grading — laser drilling reveals itself through characteristic features:
Traditional drilling:
- Straight, cylindrical channels visible as fine white tubes
- Channels run perpendicular or at steep angles to the surface
- Bright reflection in darkfield; shadow casting in transmitted light
- No natural inclusion forms a perfectly straight tube
KM (internal) drilling:
- Irregular feather networks radiating from a bleached inclusion site
- Feathers connect to the surface in patterns correlated with inclusion position
- Bleached or dissolved remnants at the centre of the fracture network
- Lack of natural geological context — the feathers exist to provide acid access, not from formation stress
Both types are identifiable by a competent gemologist using standard 10× magnification and darkfield illumination.
Value Impact
Laser-drilled diamonds trade at a discount of approximately 10% to 30% below comparable untreated stones, depending on how significantly the treatment improved the diamond's apparent clarity. This discount is considerably smaller than the 30% to 70% reduction seen with fracture filling, reflecting laser drilling's key advantages: it is permanent, the diamond receives a full GIA report, and no special care is required.
A diamond that was I1 before treatment and grades SI2 after drilling offers a price entry point below a natural SI2 — with the treatment permanently noted on the report. For buyers who prioritise visual appearance within a budget, this represents genuine value. For buyers focused on resale, the treatment note limits the secondary market.
Summary
Laser drilling is the diamond industry's established method for reaching dark inclusions beyond a cutter's options. Traditional drilling leaves a visible straight channel; KM drilling creates natural-looking fractures that serve the same access purpose. Both are permanent, both are detectable under standard magnification, and both are recorded on GIA reports. The drill hole, the bleached remnant, the notation on the report: the permanent record of a diamond whose appearance was improved by technology, not geology.
Related Reading
- Fracture Filling — the other major clarity treatment, which unlike laser drilling is impermanent and prevents a diamond from receiving a standard GIA grading report.
- Clarity Treatments Overview — how laser drilling and fracture filling fit within the broader category of clarity-enhancing interventions.
- Treatment Disclosure — the legal and ethical framework requiring sellers to disclose all diamond treatments, including laser drilling, at the point of sale.
- Care Risks for Treated Diamonds — care guidelines for treated diamonds, noting that laser drilling requires no special protocols beyond standard diamond care.