What Happens After a Lab Diamond Is Grown: Cutting, Polishing, and Certification Steps

After a lab diamond leaves the growth chamber, it goes through five distinct post-growth stages: rough assessment and planning, cleaving or laser-sawing, bruting, faceting and polishing, and finally IGI certification. Each stage shapes the stone’s optical performance and determines its graded quality. The growing part gets most of the attention, but the cutting and polishing work that follows is just as skill-intensive, and a poor cut can waste a significant share of a rough stone’s potential value before it ever reaches a jeweler.

What happens during the rough assessment stage?

The first thing a cutter does after receiving a rough lab diamond is study it carefully under magnification. Before any blade or laser touches the stone, the team needs to understand what they are working with.

Internal inclusions, growth sectors, and strain patterns are all mapped at this stage. The planner is essentially solving a geometry problem: how to extract the most valuable polished diamond from an irregular crystal while positioning any inclusions where they will have the least visual impact. Planning decisions here directly affect the final carat weight, and carat weight has the largest single effect on polished diamond price.

One specific metric the industry tracks is yield, calculated by dividing the finished polished carat weight by the original rough carat weight. A typical round brilliant cut yields somewhere in the range of 35 to 50 percent of the rough weight, which means more than half the original crystal becomes dust or off-cuts. That loss is not waste in the careless sense; it is the price of optical precision.

How does cleaving or laser-sawing divide the rough stone?

Once the cutting plan is finalized, the rough stone is marked and then divided. Traditionally this was done by cleaving along the crystal’s natural grain planes, but today most labs use laser sawing, which allows cuts that follow the optimal plan rather than the stone’s natural cleavage.

For lab diamonds specifically, the growth method matters here. CVD-grown stones may behave unpredictably during cleaving because of their layered growth structure, while HPHT stones tend to cleave more consistently due to the uniform pressure environment in which they formed. Laser cutting sidesteps most of that variability and can produce complex fancy shapes that mechanical cleaving cannot.

What is bruting, and why does it matter for round diamonds?

After the rough is divided, a round brilliant still looks nothing like a finished diamond. Bruting is the step that changes that. The stone is mounted in a lathe and spun against another diamond (or a diamond-coated wheel), grinding the girdle into a circular outline. This forms the basic silhouette that all subsequent faceting will follow.

The girdle produced by bruting is the widest part of the finished stone, and its thickness affects both durability and how light enters the pavilion. A girdle that is too thin risks chipping; one that is too thick adds weight without adding beauty. Getting it right is a judgment call made by the cutter based on the original planning dimensions.

How does faceting and polishing create a diamond’s brilliance?

This is the stage that takes the longest. Brillianteering, the final polishing phase, typically accounts for 60 to 70 percent of the total cutting time. A standard round brilliant receives 57 or 58 facets in total: table, crown facets, star facets, upper and lower girdle facets, and pavilion facets, each ground and polished on a rotating wheel coated in diamond paste.

The angles are not arbitrary. For a round brilliant, the table sits at 0 degrees, crown facets at roughly 34.5 degrees, and pavilion facets at approximately 40.75 degrees. These proportions determine how light travels through the stone, bounces off the pavilion, and returns to the eye as brightness, fire, and scintillation. A stone cut even a few degrees outside those tolerances will leak light through the bottom instead of returning it upward.

After brillianteering, the finished diamond goes through a quality control check where gemologists confirm the weight against the original yield estimate and assess symmetry, proportions, and polish under magnification. If anything falls short, the stone goes back for re-polishing before it moves forward.

What does IGI certification actually test?

Once the stone clears quality control, it is sent to an independent grading laboratory. For lab-grown diamonds sold in the United States, IGI (the International Gemological Institute) is the most widely used certifier, and all diamonds at Ouros Jewels carry IGI reports.

At IGI, multiple gemologists assess the same stone independently without sharing their grades. Color is evaluated with the diamond placed face-down under standardized lighting to remove any bias from brilliance. Clarity is assessed at 10x magnification, where inclusions and blemishes are mapped and graded from Flawless down through the Included categories. Cut grade for round brilliants is determined by comparing the stone’s actual proportions against IGI’s own research database covering brightness, fire, scintillation, and pattern.

Beyond the four Cs, the lab runs advanced spectroscopic testing to confirm the stone’s origin and growth method (CVD or HPHT), and that finding is noted on the report. Once grading is complete, a unique report number is laser-inscribed on the girdle, invisible to the naked eye but readable under a loupe. That inscription ties the physical stone to its paper report permanently.

The Cut Grade Is What Ties Every Step Together

Every stage described above feeds into a single outcome: the cut grade on the IGI report. Rough assessment determines what proportions are even possible. Cleaving preserves the weight needed to hit target dimensions. Bruting sets the girdle geometry that faceting depends on. And polishing determines whether the finished angles actually deliver the light performance the plan promised.

A buyer who focuses only on color and clarity while ignoring cut is missing the factor that most visibly affects how a diamond looks in person. An Excellent cut grade on an IGI report means every stage of the post-growth process was executed within tight tolerances. For shoppers exploring IGI-certified lab-grown diamond jewelry, that grade is the most reliable shorthand for optical quality available.

Frequently Asked Questions

How long does the cutting and polishing process take for a lab-grown diamond?

Cutting a 1-carat round brilliant typically takes between four and twelve weeks from planning through final polish. Planning and 3D modeling alone can take one to two weeks, and the brillianteering phase accounts for the majority of hands-on time. Fancy shapes like marquise or pear generally take longer because their asymmetrical outlines require more individual facet adjustments.

Does the IGI report say whether a diamond was grown by CVD or HPHT?

Yes. IGI identifies the growth method using spectroscopic testing and notes it in the comments section of the grading report upon request. CVD and HPHT diamonds are chemically identical, but the growth method affects how the rough stone behaves during cutting, so it is useful information for cutters and buyers alike.

Can a lab diamond be re-cut after it has already been certified?

It can, but re-cutting changes the stone’s physical dimensions and carat weight, which invalidates the existing IGI report. The diamond would need to be re-submitted for a new grading report after any re-cutting. Most buyers never need to do this; re-cutting is typically only considered when a stone has been chipped or when an owner wants to change the shape entirely.

Why do some lab diamonds with the same carat weight look noticeably larger than others?

Carat weight measures mass, not diameter. A poorly cut stone can carry extra weight in the pavilion or girdle, making it heavier without appearing larger face-up. A well-cut stone distributes its weight across the proper proportions, maximizing the visible surface area. This is one reason cut grade matters more than raw carat weight when comparing two stones side by side.

What does the laser inscription on a lab diamond’s girdle actually look like?

The inscription is microscopic text etched directly into the girdle surface, typically the IGI report number and sometimes the words “Lab Grown.” It is not visible to the naked eye but can be read clearly under a 10x jeweler’s loupe or a microscope. The inscription permanently links the physical stone to its grading report, making it straightforward to verify authenticity before purchase.