Introduction
When a diamond's proportions fall outside the optimal range, predictable visual defects appear: areas of darkness, unwanted reflections, or a washed-out transparency that no amount of carat weight can compensate for.
Three issues account for the vast majority of light performance problems in round brilliants: light leakage, the fish-eye effect, and the nail-head. Each has a specific cause, a distinct visual signature, and can be identified before purchase. For the proportions that define well-cut stones, see Proportions Primer.
Light Leakage
What It Is and What Causes It
Light leakage occurs when light enters through the crown but passes straight through the pavilion instead of reflecting back. The result is areas of transparency or darkness — regions that appear "dead" because they return no light.
In a well-proportioned diamond, pavilion facets trigger total internal reflection — bouncing light back inside the stone. When the pavilion angle is too shallow (below approximately 40.4°), the angle of incidence drops below the critical threshold. Light passes through instead of reflecting. The diamond becomes partially transparent.
Total depth below 59% is a strong indicator, but pavilion angle on the grading report is the definitive number — a stone can have acceptable depth but still leak if the pavilion is shallow and the crown compensates.
How to Spot It
In photos and video, leakage shows as dark or transparent patches where facets look grey and lifeless. The pattern is most visible around the stone's edges.
Retail overhead lighting can mask leakage. Tilt the diamond or view under diffuse lighting — if areas go dark or transparent, the stone is leaking. ASET (Angular Spectrum Evaluation Tool) images make leakage unambiguous: leaking areas appear white, while properly reflected light appears red or green.
Proportions to avoid: pavilion angle below 40.4°, total depth below 58.5%.
Fish-Eye Effect
What It Is and What Causes It
The fish-eye is an extreme form of light leakage in very shallow diamonds. Looking through the table, you see a circular reflection of the girdle — the stone's outer edge — visible as a white or grey ring. The pavilion acts less like a mirror and more like a lens, projecting an image of the diamond's own perimeter back through the crown.
This occurs at pavilion angles below approximately 40.0°, most pronounced in stones with total depth below 57% and table percentages above 60%. The large table window combined with a flat pavilion creates the widest possible view of the girdle reflection.
How to Spot It
The fish-eye is the easiest defect to identify. Face-up under normal lighting, a distinct ring-shaped reflection appears inside the table — a pale band clearly different from the normal facet pattern. In video, the ring remains static while a well-cut stone's reflections flash dynamically.
Fish-eye stones often appear disproportionately large face-up because shallow depth spreads weight across a wider diameter. The temptation of visual size is real, but the light performance trade-off makes it a poor bargain. See Face-Up Size vs Hidden Weight for why chasing spread at the expense of cut costs more than it gains.
Proportions to avoid: pavilion angle below 40.0°, total depth below 57%, table above 60%.
Nail-Head
What It Is and What Causes It
The nail-head is the opposite problem. Instead of light escaping through the bottom, it exits through the sides — leaving the centre dark when viewed face-up. A dark circle in the middle surrounded by edge brightness, resembling a nail head.
This forms when the pavilion angle exceeds approximately 41.2°. Light reflects off the pavilion but exits laterally through the crown's edge facets rather than returning through the table. The effect worsens with steeper angles — at 41.6° or higher, the dark centre can cover a third of the face-up area.
Steep pavilion angles correlate with excessive depth (above 63%) and smaller-than-expected diameter. A nail-head stone delivers a double penalty: it looks smaller than its weight suggests, and the visible area shows a dark centre. See What Cut Controls for how pavilion angle governs brilliance.
How to Spot It
A nail-head appears as persistent darkness in the table centre under normal lighting. It is most apparent under diffuse lighting or when the observer's head partially blocks overhead light — the dark reflection of the observer's silhouette appears disproportionately large.
In video, a nail-head stone sparkles mostly around the perimeter while the centre remains persistently dark, unlike the edge-to-edge flash of a well-cut stone.
Proportions to avoid: pavilion angle above 41.2°, total depth above 63%.
How to Protect Yourself
Check pavilion angle first. Between 40.6° and 41.0° on the grading report rules out severe versions of all three defects. See Proportions Primer.
Check total depth. The 59–62.5% range is safe. Below 58.5% signals leakage risk. Above 63% signals nail-head risk and hidden weight.
Trust the cut grade — but verify. A GIA Excellent cut grade eliminates the severe versions of all three defects. It is the single strongest safeguard available.
Examine images and video. Look for the specific signatures: transparent patches (leakage), a girdle ring in the table (fish-eye), a dark centre (nail-head). ASET or IdealScope images translate these into colour maps that make defects unambiguous.
View under multiple lighting. Move between spotlight, diffuse, and natural light. A well-cut stone performs in all three. A compromised stone reveals itself when conditions change.
Under Czech consumer protection law, sellers must substantiate claims about diamond quality. If a stone is described as well-cut, the grading report should be available. Use it.
Summary
Light leakage, fish-eye, and nail-head are predictable consequences of proportion errors — not random flaws. All three are governed primarily by pavilion angle, the most consequential proportion in a round brilliant. The optimal range of 40.6–41.0° is narrow, and deviations of less than half a degree produce visible consequences.
These defects are avoidable. A GIA Excellent cut grade, confirmed by a quick check of pavilion angle and total depth, gives you strong assurance that the stone returns light the way a diamond should — brilliance from edge to edge, no dead zones, no dark centres, no phantom reflections. The grading report is your tool. Use it.