How Lab Grown Diamonds Are Cut and Polished After Growth: The Full Process

After a lab grown diamond finishes growing, it exits the reactor as an opaque, rough crystal with no visible sparkle. Every grade on a finished certificate traces back to the cutting and polishing stages that follow. Understanding those stages tells you exactly what you are paying for when you buy a finished stone.

The rough crystal looks nothing like what ends up in a ring. It tends to be yellowish or grayish, with a waxy surface and irregular geometry. Cutting and polishing can consume roughly 50 percent of the rough’s weight, which is why the skill of the cutter matters as much as the quality of the growth process itself.

What happens to a lab grown diamond before any cutting begins?

The first thing a cutter does is study the rough, not cut it. Each crystal is scanned using 3D imaging software that maps internal inclusions, grain direction, and overall geometry. The goal is to find the cutting plan that yields the best combination of finished carat weight and optical quality, because those two goals often pull against each other.

In some cases, a technician will polish a small “window” into the rough surface to get a clearer view of interior clarity before committing to a plan. Once the plan is set, the rough may be divided into two or more pieces. This is done either by cleaving, which splits the stone along its natural crystal planes by striking a blade set into a pre-cut groove called a kerf, or by sawing, where a copper blade loaded with diamond powder cuts through the stone. Laser sawing has largely replaced mechanical sawing in modern facilities because it wastes less material and gives the cutter more control over the cut line.

How does bruting give the diamond its shape?

Once the rough is divided, each piece goes through bruting, the stage that produces the diamond’s outline shape. For a round stone, the crystal is mounted on a spinning lathe called a dopstick, and a second diamond (or a laser in more recent setups) grinds against the spinning girdle edge until the stone becomes round. The friction is intense enough that olive oil or graphite is used as a lubricant to manage heat and keep the abrasive powder in place.

Bruting creates the girdle, the narrow band that separates the crown (the top half of the finished stone) from the pavilion (the bottom half). At this stage the diamond has its basic silhouette but no facets at all. It looks like a frosted lens. For fancy shapes such as ovals, pears, or marquises, the bruting process is more complex, and cutters often rely on laser shaping rather than the rotary method used for rounds.

What is the difference between blocking and brillianteering?

Blocking is where the diamond starts to look like a diamond. The stone is set into a holder called a tang and lowered onto a rotating iron wheel, called a scaife, that spins at roughly 3,000 revolutions per minute and is charged with diamond powder mixed into a lubricant. The cutter polishes the 16 main facets: eight on the pavilion and eight on the crown. Blocking the pavilion at consistent angles causes those facets to converge at a point, forming the culet. Blocking the crown forms the flat top surface, the table facet.

Brillianteering follows, and this is where the remaining facets are added. For a standard round brilliant, that means polishing 8 star facets, 16 upper girdle facets, 8 pavilion facets, and 16 lower girdle facets, for a total of 58 facets across the finished stone. Each facet must be cut at a precise angle relative to the others, because even a fraction of a degree of error changes how light travels through the stone. A well-executed brilliant cut causes light entering the crown to bounce off the pavilion facets and return upward through the table. A poorly angled pavilion lets that light leak out the bottom instead.

How does final polishing affect light performance?

After all facets are placed, the diamond goes through a dedicated polishing pass. The same scaife wheel is used, but the focus shifts from creating new facets to smoothing the surfaces already cut. Polish removes microscopic surface irregularities left by the cutting process. Under 10x magnification, an unpolished facet shows fine parallel lines called polishing lines. A well-polished facet is optically flat, meaning light reflects off it cleanly rather than scattering.

Polish grade appears on the IGI certificate alongside symmetry, and both feed into the overall cut grade. IGI awards an Excellent polish grade only when facet surfaces show no polishing lines, abrasions, or surface irregularities under 10x magnification, and the proportions of a round brilliant must fall within tight ranges: a table percentage between roughly 54 and 60 percent and a total depth between 59 and 62.5 percent to reach Excellent cut. A stone with sound proportions but a Fair polish grade will still show reduced brilliance, because rough facet surfaces scatter some of the light that should be returning to the eye. Polishing is the stage that determines whether the cutting work that came before it is actually visible.

Why does the cut grade on an IGI certificate matter more than most shoppers realize?

Certification is the final stage of manufacturing, coming after polishing is complete. IGI, which holds approximately 65 percent of the lab grown diamond certification market, evaluates each finished stone on the 4Cs: carat weight, color on the D-to-Z scale, clarity at 10x magnification, and cut. The cut grade itself is a composite of three measurements: proportions (the angles and percentages of each part of the stone), symmetry (how precisely the facets align with each other), and polish (the surface quality of each facet).

An IGI report for a lab grown diamond also explicitly states the growth method, CVD or HPHT, and discloses any post-growth treatment such as annealing. That disclosure matters because some CVD-grown diamonds are annealed after growth to improve color, and a buyer deserves to know. The laser inscription on the girdle ties the physical stone to its specific report number, so the certificate travels with the diamond permanently.

For shoppers comparing stones online, the cut grade is often the most actionable number on the report. Two stones with identical color and clarity grades can look dramatically different in a ring if one was cut to Excellent proportions and the other was not.

The Cut Is What the Eye Actually Sees

Every stage described above, from the 3D planning scan through bruting, blocking, brillianteering, and final polishing, exists to serve one outcome: a finished stone that returns light to the viewer’s eye in a predictable, beautiful pattern. Growth method sets the raw material; cut is what the eye actually sees in the finished ring.

For shoppers ready to put this knowledge to use, the Lab Grown Diamonds collection at Ouros Jewels shows how different cut styles translate into finished stones, each with its IGI certificate documenting the grades the cutting process produced.

Frequently Asked Questions

Does the cutting process for lab grown diamonds differ from cutting natural diamonds?

No. The physical process is identical because the material is identical: both are pure carbon in a cubic crystal structure. The same bruting, blocking, brillianteering, and polishing stages apply. The only practical difference is that lab grown rough is often more predictable in shape and consistency than mined rough, which can reduce planning time.

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

Cutting typically removes around 50 percent of the rough stone’s weight. A 2-carat rough crystal will commonly yield a finished stone between 0.90 and 1.10 carats, depending on the shape chosen and how the cutter balances weight retention against optical performance. Fancy shapes like ovals and pears tend to retain slightly more weight than rounds.

What does the polish grade on an IGI certificate actually measure?

Polish grade reflects the smoothness of each facet surface after the final polishing stage. Grades run from Excellent down through Very Good, Good, Fair, and Poor. An Excellent polish grade means the facet surfaces are optically flat under 10x magnification, with no visible polishing lines or surface irregularities that would scatter light.

Can a lab grown diamond be recut or repolished after purchase?

Yes, though it is uncommon. A stone with a minor chip on a facet edge or a polish grade the buyer wants to improve can be sent back to a cutter. Recutting will reduce carat weight, which changes the stone’s value, and a new IGI report would be needed afterward to reflect the updated measurements and grades.

Why do some lab grown diamonds show brown or gray tones before post-growth treatment?

CVD growth sometimes introduces structural defects called vacancy clusters that absorb certain wavelengths of light, producing a brownish or grayish tint in the rough crystal. Annealing, which involves heating the stone under controlled conditions after growth, rearranges those defects and typically restores a whiter, more colorless appearance. IGI discloses this treatment on the grading report.