Introduction
On every GIA Diamond Grading Report, between the 4Cs panel and the clarity plot, sits a small line drawing of the diamond in profile. It is not decorative. This is the proportions diagram — a scaled schematic that labels every measurable angle, percentage, and thickness that defines the stone's geometry.
The diagram takes what would otherwise require a calculator and a gemological glossary and makes it visual. Crown angle, pavilion angle, table percentage, total depth, girdle thickness, culet size, star length, and lower-half facet length are all printed directly on the drawing, each pointing to the feature it describes. For round brilliants, these numbers feed into the overall cut grade. For fancy shapes — which receive no cut grade from GIA — this diagram is the closest thing to a cut assessment the report provides.
This article explains each element of the proportions diagram, what the numbers mean, and how to use them to evaluate a diamond's light performance at a glance. For the underlying science of how these proportions affect brilliance, fire, and scintillation, see Proportions Primer. For where this diagram fits in the overall report layout, see How to Read a Report.
Key Points
What the diagram shows
The proportions diagram is a two-dimensional cross-section of the diamond viewed from the side — a profile silhouette. It divides the stone into three visible zones:
Crown — the upper portion, from the table to the girdle. The diagram shows the table facet as a flat line at the top, with the crown facets angling down to the girdle. Two measurements are labelled here: table percentage (the width of the table relative to the average girdle diameter) and crown angle (the angle between the bezel facets and the girdle plane, in degrees).
Girdle — the thin band at the widest point, shown as a narrow strip between crown and pavilion. The diagram labels girdle thickness as a descriptive range (e.g., "Thin to Medium" or "Medium to Slightly Thick").
Pavilion — the lower portion, from the girdle to the culet. The diagram labels the pavilion angle (the angle between the pavilion main facets and the girdle plane) and the culet size (from None to Large).
Total depth percentage is printed alongside the profile, measuring the full height from table to culet as a percentage of the average girdle diameter.
Two additional measurements appear on detailed reports: star length and lower-half facet length, both expressed as percentages. These describe secondary facet proportions that influence the diamond's scintillation pattern — how sparkle is distributed across the face.
Reading each measurement
Table percentage. Printed at the top of the diagram. For round brilliants, the GIA Excellent range centres on 54–58%. A table below 53% reduces light intake; above 60%, the crown facets lose area needed for fire. On fancy shapes, table percentage ranges vary by shape — ovals and cushions typically run wider (56–62%) without penalty.
Crown angle. Labelled on the sloped crown profile. The optimal range for round brilliants is 34–35°. This angle governs how much spectral dispersion (fire) exits the crown facets. A crown below 33° produces a flat look with reduced fire. Above 36°, light starts redirecting sideways rather than toward the viewer. See Crown Angle for the full explanation.
Pavilion angle. Labelled on the lower profile. This is the single most influential proportion for brilliance. The optimal range for rounds is narrow: 40.6–41.0°. Below 40.4°, light leaks through the bottom of the stone. Above 41.2°, light exits sideways, creating a dark centre visible face-up. A fraction of a degree matters — GIA measures this to the nearest tenth. See Pavilion Angle.
Total depth percentage. Shown beside the full profile. For round brilliants, 59–62.5% balances light performance with face-up size. Depth above 63% hides carat weight below the setting line — the diamond weighs more but looks smaller. Below 58%, the stone is too shallow for reliable internal reflection. See Total Depth Percentage.
Girdle thickness. Labelled at the girdle band. Descriptive terms range from Extremely Thin to Extremely Thick. The optimal range is Thin to Slightly Thick. Extremely Thin girdles chip easily during setting. Extremely Thick girdles add invisible weight that inflates carat cost without increasing visual size. GIA reports a range (e.g., "Thin to Medium") because the girdle is rarely uniform around the stone's circumference. See Girdle Thickness.
Culet size. Labelled at the bottom point. Modern round brilliants almost universally have a "None" (pointed) culet. A visible culet — Small or larger — appears as a dark spot through the table when viewed face-up. On vintage or older cuts, a small culet is normal and not a defect. On new production, anything larger than Very Small warrants a question to the seller. See Culet Size.
Star length. Expressed as a percentage of the distance from the table edge to the girdle. Typical values fall between 45% and 65%. Shorter stars (below 50%) create broader, bolder flashes of light. Longer stars (above 55%) break the pattern into smaller, more numerous sparkles. Neither is inherently better — it is a matter of visual preference.
Lower-half facet length. Expressed as a percentage of the distance from the girdle to the culet. Typical values range from 75% to 80%. Shorter lower halves produce broader light flashes; longer ones create finer scintillation. Combined with star length, this measurement defines the diamond's sparkle character — whether it reads as bold and blocky or fine and lively.
Using the diagram without a cut grade
For round brilliants, GIA synthesises all proportions into a single cut grade (Excellent through Poor). But for fancy shapes — oval, cushion, emerald, pear, princess, marquise, radiant, asscher — no overall cut grade is assigned. The proportions diagram becomes your primary evaluation tool.
Without a cut grade, compare the listed values against known performance ranges for the specific shape. These ranges are not standardised by GIA in the same way as for rounds, but experienced gemologists and reputable dealers work from well-established benchmarks. For example, an oval with a table of 56–60%, a depth of 58–62%, and balanced crown and pavilion angles is generally well-proportioned.
The practical approach: start with pavilion angle and table percentage. If the pavilion angle falls in the range associated with good light return for that shape, and the table percentage is not extreme, the geometry is likely sound. Then check depth percentage to ensure weight is not hidden. Finally, review girdle thickness for durability.
This is not a substitute for seeing the diamond in person or in high-resolution imagery — proportions alone cannot capture optical symmetry or light leakage patterns. But it is a reliable first filter. A fancy-shape diamond with proportion values well outside typical ranges is unlikely to perform well, regardless of how appealing its other grades look.
Common red flags on the diagram
Certain combinations visible on the proportions diagram signal problems worth investigating:
- Pavilion angle above 41.4° combined with depth above 63%. The stone is likely deep-cut with a dark centre (nail-head effect). Weight is hidden below the girdle.
- Table above 62% with crown angle below 32°. A flat, "pancake" profile. Brightness may be acceptable, but fire will be minimal — the crown simply lacks the geometry to disperse light.
- Girdle described as "Very Thick to Extremely Thick." Significant hidden weight. Compare the millimetre diameter against other diamonds of the same carat weight to see how much face-up size you are losing. See Measurements Box.
- Culet listed as "Medium" or "Large" on a modern round brilliant. Unusual for current production. The culet may be visible face-up as a dark circle. Ask for imagery.
Czech Consumer Note
Czech buyers purchasing online — particularly from international sellers — should pay close attention to the proportions diagram when evaluating fancy-shape diamonds. Since these shapes carry no GIA cut grade, the diagram is the most objective tool available for assessing cut quality remotely. Under EU consumer protection regulations applicable in the Czech Republic, sellers must provide accurate product descriptions. If a seller describes a fancy-shape diamond as "excellent cut" or "ideal proportions" without a laboratory-assigned cut grade to support that claim, the proportions diagram on the actual report is your verification tool. Compare the numbers yourself before committing, especially on higher-value purchases where the price difference between well-cut and poorly-cut stones of the same carat weight can exceed tens of thousands of CZK.
Summary
The proportions diagram is a map of the diamond's geometry, printed in plain sight on every grading report. It labels table percentage, crown angle, pavilion angle, total depth, girdle thickness, culet size, star length, and lower-half facet length — every measurement that determines how light moves through the stone.
For round brilliants, these numbers underpin the cut grade. For fancy shapes, they are the cut grade — or the closest substitute available. Learning to read the diagram takes minutes. The two numbers that matter most: pavilion angle (does light reflect back or leak away?) and table percentage (is there room for both brilliance and fire?). Start there, check depth and girdle for hidden weight and durability, and you have a reliable first assessment of any diamond's proportions — before you ever see it in person.