What Happens to a Lab Grown Diamond After It Leaves the Growth Chamber

Once a lab grown diamond leaves its growth chamber, it enters a second, equally demanding pipeline: sorting, optional color correction, planning, cutting, polishing, and independent certification. Each stage directly shapes the stone’s final grade and value. The growth phase produces a rough crystal. Everything that happens next determines whether that crystal becomes a D-color, Excellent-cut gem or a lower-tier commercial stone.

That gap matters for shoppers. Two diamonds grown in the same reactor on the same day can finish with very different certificates, depending on decisions made at the cutting wheel and the grading table. Understanding those stages helps you read a grading report with real confidence rather than treating it as a formality.

Does a rough lab grown diamond need treatment before it can be cut?

Sometimes, and the answer depends on which growth method was used. Diamonds grown by Chemical Vapor Deposition (CVD) commonly come out of the chamber with a brownish or grayish tint caused by lattice dislocations and nitrogen-vacancy defects that form during deposition. Most CVD stones intended for gem-quality jewelry go through an HPHT annealing step before cutting: the already-formed rough is placed back into a high-pressure, high-temperature environment, which reorganizes those defects at the atomic level and permanently improves the color. Nothing is added to the stone. The lattice is corrected, and the result is stable.

HPHT-grown diamonds tend to emerge with fewer color issues and generally skip this correction step, though they sometimes carry trace metal inclusions from the catalyst used during growth.

Whether or not treatment was applied, a full IGI grading report will note it explicitly in the comments section, so the buyer can see exactly what they are purchasing. That transparency is part of what makes certification worth requiring.

How are rough lab grown diamonds sorted and planned before cutting begins?

Sorting comes before any cutting wheel touches the stone. Planners evaluate each rough crystal for likely color, clarity, and cutting yield, sometimes polishing a small window on the surface to get a clearer read on internal characteristics. From there, the rough is categorized by size and quality, which determines which cutting facility handles it and what shape the finished stone will target.

Planning follows sorting, and it is where the economics of the stone get decided. The goal is to maximize the polished carat weight from each piece of rough, because carat weight drives price more than any other single variable. Modern cutting facilities use 3D scanning and computer modeling to map inclusions and calculate the optimal cut strategy before a laser or blade ever contacts the crystal. A rough stone that weighs 1.00 ct might yield a polished diamond of around 0.39 ct, a yield of roughly 39%, once cutting removes 40 to 60 percent of the original crystal mass.

Almost all rough diamonds need to be divided into more than one section to achieve the best overall outcome, and that division happens through cleaving along natural crystal planes or laser sawing in other directions.

What does the cutting and polishing process actually involve?

Cutting a lab grown diamond follows the same sequence used for mined diamonds, because the material is physically identical. After the rough is divided, bruting rounds the stone into its basic outline by pressing it against another diamond or a diamond-coated wheel. This establishes the girdle shape and creates the starting geometry for faceting.

Faceting is the stage that determines optical performance. Skilled cutters place each facet at precise angles to control how light enters, bounces internally, and exits the stone. A round brilliant, for example, has 57 or 58 facets, each positioned to maximize brightness and fire. A poorly angled crown or pavilion does not just look dull: it drops the cut grade on the final certificate, which directly affects resale value.

Polishing follows faceting, using a rotating wheel to bring each facet to a mirror finish. At this stage, symmetry and polish grades are assessed, and any facet that falls short of the target gets reworked. The process is slow and precise. A single facet polished at the wrong angle means the stone goes back.

How does IGI certification work, and what does the report actually tell you?

After polishing, the finished stone goes to an independent gemological laboratory for grading. For lab grown diamonds in 2026, IGI is the standard: it grades on the full 4Cs scale (cut, color, clarity, and carat weight) and has done so since it pioneered lab grown diamond certification in 2005.

The grading process evaluates color in a standardized viewing environment, assesses clarity under magnification based on the visibility, size, number, and location of internal characteristics, and compares the stone’s proportions against IGI’s studies of brightness, fire, and scintillation to assign a cut grade. Beyond the 4Cs, the report includes the stone’s precise measurements, fluorescence grade, symmetry and polish grades, a clarity diagram, and explicit notation of the growth method (CVD or HPHT) and whether post-growth treatment was applied.

Each certified stone receives a laser inscription on its girdle that matches the report number, making the certificate verifiable directly on IGI’s website. GIA, for context, moved to a simplified two-tier system for lab grown diamonds in October 2025, classifying stones as “Premium” or “Standard” rather than assigning specific grades like D color or VVS2 clarity. IGI’s full 4Cs grading is what allows a buyer to compare stones accurately and know precisely what they are paying for.

The Cut Grade Is What the Growth Chamber Cannot Give You

A lab grown diamond’s origin story ends the moment it leaves the reactor. What the certificate says about it is determined entirely by what happens next. Color treatment, cutting geometry, and grading rigor are the three variables that separate a well-finished stone from one that only looks similar on paper.

For shoppers, the practical takeaway is straightforward: require a full IGI report, check that the cut grade reaches at least Very Good, and verify the report number online before purchase. The growth method matters less than most buyers assume. The cut grade matters more than most realize.

Ouros Jewels carries IGI-certified lab grown diamonds graded on the full 4Cs scale, with report details you can verify independently before committing to a stone.

Frequently Asked Questions

Does post-growth HPHT treatment make a CVD lab grown diamond lower quality or less genuine?

No. Post-growth HPHT annealing corrects structural defects at the atomic level without adding or removing material. The color improvement is permanent and does not affect the diamond’s hardness, optical properties, or durability. A treated stone with a D color grade is, for all practical purposes, a D color diamond. IGI reports disclose treatment status clearly.

How much weight does a lab grown diamond lose during cutting and polishing?

Cutting and polishing typically removes 40 to 60 percent of the original rough crystal’s mass. A 1.00 carat rough stone might yield a finished polished diamond of around 0.35 to 0.50 carats, depending on the shape chosen and the location of any inclusions the cutter needs to work around. This weight loss is why polished lab grown diamonds cost more per carat than the rough material.

What is the difference between an IGI lab grown diamond report and a GIA report in 2026?

IGI grades lab grown diamonds on the full 4Cs scale, assigning specific grades like E color or VS1 clarity. Since October 2025, GIA replaced specific 4Cs grading for lab grown stones with a two-tier system that classifies diamonds only as “Premium” or “Standard.” For buyers who want to compare stones precisely, IGI’s grading gives significantly more usable information.

Can a jeweler tell if a lab grown diamond has been post-growth treated just by looking at it?

No. Post-growth HPHT treatment is not visible to the naked eye or under standard jeweler’s loupe magnification. Identifying treatment requires spectroscopic testing, specifically photoluminescence spectroscopy, which is only available at a gemological grading laboratory. This is one reason why requiring an IGI report, which explicitly notes treatment status, matters when buying a lab grown diamond.

Does the cut grade on an IGI report affect a lab grown diamond’s resale value?

Yes, meaningfully. Cut grade governs how a diamond handles light, and a stone graded Good or Fair will look noticeably duller than one graded Very Good or Excellent at the same color and clarity. Resale buyers and trade buyers price cut quality into their offers. An Excellent cut stone at a lower color grade will often hold value better than a higher-color stone with a mediocre cut.