Most people imagine that selecting a diamond begins with colour or carat weight. In practice, the first thing a trained eye reaches for is cut quality. Cut is the only one of the so-called four Cs that is shaped entirely by human decision rather than by the growth conditions of the crystal. It governs how light travels through the stone, how it returns to the eye, and whether the diamond looks alive or simply heavy. A diamond can carry an admirable colour grade and a respectable carat weight and still disappoint, because a poorly cut stone leaks light, looks flat in ordinary lighting, and never quite earns its size.
This is the part of the process that rarely gets explained. When a lapidary or a trained buyer screens a parcel of lab-grown diamonds, the work is far closer to rejection than to selection. For every stone that earns a place on a shortlist, several are set aside. This article walks through how that screening is done: the proportions that are measured, the polish and symmetry that are inspected under magnification, the question of eye-cleanliness, and the concrete faults that send a stone back. It is meant to demystify the discipline behind our quality standards, so that you understand what considered selection actually involves.
Why cut quality sits above the other characteristics
A diamond is a lens. Light enters through the table and crown, strikes the pavilion facets, and ideally reflects back toward the viewer rather than escaping through the bottom of the stone. When the angles are correct, that returned light reads as brightness, contrast, and the flashes of spectral colour known as fire. When the angles are wrong, light passes straight through, and the diamond shows dull or glassy patches that no colour grade can rescue.
Because lab-grown diamonds are chemically, physically, and optically identical to mined diamonds, the rules of light behaviour are exactly the same. A lab-grown stone is grown in controlled conditions, but it is still cut by a human lapidary working rough against a wheel, making the same trade-offs between weight retention and optical performance. A cutter who prioritises holding onto carat weight will often produce a stone that is too deep or too shallow. That is the first thing screening is designed to catch.
This is why cut is assessed before anything else. A faultless colour grade cannot compensate for a stone that returns light poorly. Our position, set out in the cut guide, is that cut quality is the foundation everything else rests on.
Reading the proportions
The screening begins with measurements, because proportions are objective and can be checked against the certificate before the stone is ever examined by eye. The key figures sit on every IGI or GIA report, and a trained reader knows the ranges that tend to produce strong light return in a round brilliant.
Table percentage
The table is the large flat facet on the top of the stone. Expressed as a percentage of the diameter, it controls how much light enters and how the crown distributes it. A table that is too large floods the stone with brightness but starves it of fire, giving a watery, washed-out look. A table that is too small can darken the centre.
Depth percentage
Total depth is the height of the stone relative to its diameter. A stone cut too deep hides weight beneath the girdle, so a one carat diamond looks smaller from above than its weight suggests, and light escapes through the pavilion. A stone cut too shallow leaks light through the bottom and shows a dark, glassy zone known as a fish-eye.
Crown and pavilion angles
The crown angle and pavilion angle work as a pair. The pavilion angle in particular has very little tolerance: a deviation of a degree or two from the ideal is enough to undermine light return. These two angles together are the single most important determinant of how a round brilliant performs.
The following ranges are a working reference for a round brilliant cut. They are guides rather than absolute thresholds, and a skilled assessment always reads them together rather than in isolation, because a borderline figure in one measure can be offset by strength in another.
| Proportion | Working range (round brilliant) |
|---|---|
| Table | 53 to 58 per cent |
| Total depth | 59 to 62.5 per cent |
| Crown angle | 34 to 35 degrees |
| Pavilion angle | 40.6 to 41 degrees |
| Girdle | Thin to slightly thick, never extremely thin or extremely thick |
| Culet | None to very small |
A stone whose numbers fall outside these ranges is not automatically rejected, but it is flagged for closer inspection. Often the measurements alone tell the story. A round brilliant with a 64 per cent depth and a steep pavilion will be set aside before anyone reaches for a loupe, because the geometry guarantees light loss.
Inspecting polish and symmetry
Proportions describe the intended shape. Polish and symmetry describe how faithfully the lapidary executed it. Both are graded on the certificate, but a careful screening confirms them under magnification rather than trusting the grade alone.
Polish
Polish refers to the surface finish of each facet. Under ten times magnification, a poorly polished facet shows fine parallel lines from the polishing wheel, faint burn marks from heat, or pitting. These surface faults scatter light at the point of entry and exit, dulling the stone in a way that is easy to miss in a photograph but visible in the hand. Anything below a Very Good polish grade is treated with suspicion, and any visible polish lines on a major facet are grounds for rejection.
Symmetry
Symmetry is the precision of the cut: whether facets meet cleanly, whether the table is centred, whether the eight main facets of a round brilliant are even in size and alignment. Common symmetry faults include a table that sits off-centre, a culet pointing away from the middle, facets that fail to meet at a single point, and a girdle that wavers in thickness around the stone. Misaligned facets break up the pattern of light return and can produce a visible darkening on one side. A stone with strong proportions but weak symmetry will still look uneven, so symmetry is checked with the same care as the angles.
The question of eye-cleanliness
Clarity grades describe what an examiner sees under ten times magnification. Eye-cleanliness describes what you see at normal viewing distance with the unaided eye, and the two are not the same thing. A stone can carry a modest clarity grade and still be eye-clean, while another with a higher grade might have a single inclusion positioned where it catches the light.
Screening for eye-cleanliness means examining where an inclusion sits, not just whether it exists. The faults that matter most are:
- A dark crystal or cloud sitting directly under the table, where it is most visible from above
- An inclusion near the centre of the stone rather than tucked toward the girdle, where a setting could conceal it
- A feather, which is a small internal fracture, positioned close to the girdle or culet where it could affect durability under stress
- A cloud of tiny pinpoints dense enough to give the stone a hazy or milky appearance, which can occur in some lab-grown material and dampens brightness across the whole stone
Our screening sets a minimum clarity of VS1, which means inclusions are minor and require effort to locate under magnification. But the grade alone is not the decision. A VS1 stone with a single inclusion sitting plainly under the table will be set aside in favour of one whose inclusions are positioned where they do no visible harm.
What gets a stone rejected
It helps to be concrete about the faults that end a stone's candidacy. None of these are exotic. They are the ordinary results of cutting decisions that favoured weight or speed over performance.
- Excess depth. A pavilion cut too deep to retain carat weight, leaving the face-up size smaller than the weight implies and light escaping through the bottom.
- A shallow spread. The opposite fault, where the stone is cut wide and flat, producing a fish-eye reflection of the girdle visible through the table.
- A steep or off pavilion angle. Even a degree or two of deviation noticeably weakens light return, regardless of how the rest of the numbers read.
- An extremely thin girdle. A knife-thin girdle chips easily during setting and wear, and is a durability risk rather than merely a cosmetic one.
- Visible polish lines. Parallel polishing marks on a major facet that scatter light and dull the surface.
- Poor symmetry. An off-centre table or misaligned facets that produce uneven brightness and a visible dark zone.
- A centred inclusion. A dark or reflective inclusion under the table that the eye finds at normal distance.
- A milky or hazy appearance. A dense cloud of pinpoints that lowers brightness across the whole stone, sometimes described as a lack of transparency.
The pattern across all of these is the same. A trained eye is looking for reasons to say no. The discipline is not in admiring a beautiful stone, which is the easy part, but in rejecting the many that fall short before they are ever offered. This is why considered selection takes time, and why a shortlist is genuinely short.
Why this matters for a sourced-to-order model
This screening discipline is the reason our model works the way it does. Rather than holding a fixed inventory and asking you to choose from whatever happens to be in stock, we source each stone to your brief and run it through this assessment before it is confirmed. You can read more about that in how we source. It means the stones that reach you have already survived the rejections, and that the certificate you receive describes a stone whose cut, polish, symmetry, and eye-cleanliness have been checked rather than assumed.
The result is a narrower set of choices, which is the point. A short list of well-cut, eye-clean, independently certified stones is more useful than a long list that includes stones a lapidary would quietly pass over.
Frequently asked questions
Is cut quality different for lab-grown diamonds?
No. A lab-grown diamond is chemically, physically, and optically identical to a mined one, and it is cut by the same hands using the same wheels. The proportions, polish, and symmetry that make a mined diamond perform well are exactly the proportions, polish, and symmetry that make a lab-grown diamond perform well. The screening is identical.
Can a high clarity grade make up for a weak cut?
No. Clarity and cut measure different things. Clarity describes internal inclusions, while cut describes how light moves through the stone. A flawless stone that is cut too deep or too shallow will still leak light and look dull, because no internal cleanliness can correct a geometry that sends light out the bottom of the diamond rather than back to your eye.
What does eye-clean actually mean?
Eye-clean means that at a normal viewing distance, with the unaided eye, you cannot see any inclusion. It depends not only on the clarity grade but on where the inclusions sit. An inclusion tucked near the girdle may be invisible, while a smaller one directly under the table may catch the eye. Our screening assesses position, not just grade.
Why reject so many stones?
Because most cutting decisions involve a trade-off, and many of those trade-offs favour carat weight over light return. A stone cut to hold weight often sacrifices the angles that make it bright. Setting these stones aside is the only way to ensure that what reaches you is genuinely well cut rather than simply heavy.
If you would like to see the stones that have passed this screening, you can view the full collection, or read more about the standards behind every selection on our quality page.