Introduction
A lab-grown diamond does not emerge from its growth chamber ready for a ring. The raw crystal — whether HPHT-grown or CVD-grown — requires processing before it becomes a polished gemstone. For CVD diamonds in particular, this processing includes a critical intermediate step: post-growth treatment to correct the brown colour that rapid growth produces.
This article covers the treatments applied after growth, the cutting and polishing process that shapes the final stone, and the laser inscription that permanently identifies it as laboratory-grown.
Post-Growth HPHT Treatment of CVD Diamonds
Why Treatment Is Necessary
Most CVD diamonds grown at commercially viable speeds develop a brown tint during growth. The colour results from vacancy clusters — empty spaces in the crystal lattice where carbon atoms failed to deposit cleanly during rapid layer-by-layer deposition. These vacancy clusters absorb light across the visible spectrum, producing the brownish appearance.
Slower growth reduces these defects, but at the cost of production efficiency. The industry's solution is to grow quickly and treat afterward.
The Treatment Process
Post-growth HPHT treatment subjects the CVD crystal to conditions that exceed its original growth environment:
- Temperature: Above 1,600 °C — higher than the CVD growth temperature of 700–1,000 °C
- Pressure: High pressure, comparable to HPHT growth conditions
At these extreme conditions, the vacancy clusters that cause brown colour are mobilised and annealed — they either collapse, migrate, or recombine in ways that eliminate their light-absorbing effect. The result is a shift from brown toward colourless or near-colourless.
Prevalence
According to GIA data, approximately 80 % of CVD diamonds submitted for grading since 2020 show evidence of post-growth HPHT treatment. This makes it the norm rather than the exception. The treatment is disclosed on the GIA Laboratory-Grown Diamond Report, typically with a notation that the stone "may include post-growth treatments to change the color."
Detection
Gemological laboratories can detect post-growth HPHT treatment through spectroscopic analysis. The treatment alters specific defect centres in the crystal — some are destroyed, others are created — producing a spectroscopic fingerprint that differs from untreated CVD or as-grown material. This is part of the advanced identification workflow described in Spectroscopy Overview.
Post-Growth Colour Modification of HPHT Diamonds
While CVD diamonds primarily undergo HPHT treatment to remove brown colour, HPHT-grown diamonds can be further processed to create fancy colours:
- Irradiation followed by annealing can produce pink, green, blue, and other colours from colourless or near-colourless HPHT starting material
- The irradiation introduces colour centres (lattice defects that absorb specific wavelengths), and the annealing stabilises them
These treatments are disclosed on grading reports and represent a separate category from the brown-removal HPHT treatment applied to CVD stones.
Cutting and Polishing
Same Process, Same Equipment
Once any post-growth treatment is complete, lab-grown diamonds are cut and polished using exactly the same equipment, techniques, and expertise as natural diamonds. There is no separate manufacturing line. Diamond cutters in India, Israel, Belgium, and elsewhere process both categories of rough on the same wheels with the same diamond-impregnated polishing compounds.
The cutting process follows the same principles:
- Planning — analysing the rough crystal to determine the optimal cut geometry for maximum yield, beauty, and value
- Cleaving or sawing — separating the rough into workable pieces
- Bruting — shaping the girdle outline
- Polishing — cutting and polishing each facet to precise angles
Differences in Rough Shape
The primary difference is the shape of the starting material. CVD rough is tabular (flat plates), which suits certain cutting orientations. HPHT rough is cuboctahedral, which may offer different planning options. Natural rough is typically octahedral. These shape differences influence which cuts are most efficient from each type of rough, but the final polished stones are identical in facet structure and light performance.
Laser Inscription
The "Laboratory-Grown" Mark
The GIA inscribes "Laboratory-Grown" on the girdle of every lab-grown diamond it grades. This microscopic laser inscription is invisible to the naked eye but readable under magnification. It serves as a permanent, non-removable identification marker linking the stone to its grading report.
The inscription typically includes:
- The text "Laboratory-Grown"
- The GIA report number
For HPHT diamonds, the girdle inscription may note the growth method. The inscription provides a point-of-sale verification tool: any jeweller with a loupe or microscope can confirm the stone's laboratory origin.
Other Laboratories
IGI and other grading laboratories also inscribe lab-grown diamonds, following similar conventions. The specific wording varies by laboratory, but the principle is consistent: permanent girdle marking that identifies the stone as laboratory-grown.
Frequently Asked Questions
Are most lab-grown diamonds treated after growth?
Yes. Approximately 80 % of CVD diamonds submitted to the GIA have undergone post-growth HPHT treatment. This treatment removes the brown colour caused by rapid growth and is standard industry practice.
Does post-growth treatment make the diamond less "real"?
No. The diamond remains chemically and physically identical to any other diamond. The treatment modifies lattice defects that affect colour — similar in principle to how heat treatment is applied to many natural gemstones. The treatment is disclosed on the grading report.
Can you remove the "Laboratory-Grown" inscription?
Technically, a girdle can be repolished to remove an inscription, but this would reduce carat weight and alter the stone's measurements. Ethical practice requires maintaining the inscription, and any discrepancy between a stone and its report would raise immediate red flags during resale or appraisal.
Is cutting a lab-grown diamond different from cutting a natural one?
The process is identical — same equipment, same techniques, same expertise. The difference is the shape of the rough: CVD crystals are tabular, HPHT crystals are cuboctahedral, and natural crystals are typically octahedral. This affects planning but not the final quality of the cut.
Summary
Post-growth processing transforms a raw lab-grown crystal into a market-ready gemstone. For CVD diamonds, this typically includes HPHT treatment at temperatures above 1,600 °C to remove the brown colour caused by rapid growth — a treatment applied to roughly 80 % of CVD stones submitted to the GIA. After any colour treatment, lab-grown diamonds are cut and polished using the same equipment and methods as natural diamonds, then laser-inscribed with "Laboratory-Grown" on the girdle as a permanent identification marker. All treatments are disclosed on the grading report.