Cut is what brings a diamond to life. It determines how effectively the facets return light to your eye and, in turn, how much brightness, fire and sparkle you see.

That makes cut one of the most important decisions when choosing a diamond. A larger, higher-colour or higher-clarity diamond can still appear dull if its proportions and finish do not work together. A beautifully cut diamond, by contrast, can look bright, lively and wonderfully balanced.

This guide explains what diamond cut really means, how GIA cut grades work and what to look for beyond the grading report.

What does diamond cut mean?

Diamond cut is often confused with diamond shape. Shape describes the outline – round, oval, pear, cushion or emerald, for example. Cut quality describes how well the diamond has been designed and faceted.

The angles, proportions, symmetry and polish of those facets influence the way light behaves inside the stone. Good cutting is not simply about making every diamond conform to one set of measurements. It is about achieving an attractive balance of light return, contrast, pattern and spread.

The three visual effects of diamond cut

GIA describes three appearance factors that contribute to a diamond’s face-up beauty:

• Brightness: the white light reflected from the diamond, including light returned from its surface and interior.
• Fire: flashes of spectral colour created when white light is dispersed.
• Scintillation: the pattern of light and dark areas, and the flashes of sparkle seen as the diamond, light or viewer moves.

The most beautiful diamond is not necessarily the one with the greatest amount of any single effect. What matters is the balance and character you find appealing.

How GIA diamond cut grades work

For natural, standard round brilliant diamonds in the D-to-Z colour range, GIA uses five cut grades:

• Excellent
• Very Good
• Good
• Fair
• Poor

The system considers seven components: brightness, fire, scintillation, weight ratio, durability, polish and symmetry. Together, these assess face-up appearance, design and craftsmanship.

An important distinction is that GIA’s overall cut grade applies to standard round brilliants within the scope of its system. Fancy shapes such as ovals, pears, cushions, emerald cuts and marquises do not receive an overall GIA cut grade. They are assessed using their measurements, proportions, polish, symmetry and – most importantly – their appearance in person or on high-quality video.

GIA also changed its service for eligible colourless to near-colourless laboratory-grown diamonds in October 2025. Its current Laboratory-Grown Diamond Quality Assessment uses the overall classifications Premium and Standard, rather than presenting the traditional natural-diamond grading nomenclature in the same way. Always read the actual report supplied with the stone and check which laboratory issued it.

Is Excellent always better than Very Good?

An Excellent cut grade is a strong starting point, but it is not a guarantee that every person will prefer that diamond. There is a range of proportion combinations within each grade, and two Excellent diamonds may show different patterns, contrast or visual spread.

Likewise, a well-selected Very Good diamond may be beautiful and represent excellent value. The difference is not automatically invisible, nor is the most expensive grade automatically the right choice. Compare diamonds in more than one lighting environment and look at the whole stone, not only the word on the report.

At Xennox, our preference is to begin with strong technical credentials and then make the final decision with our eyes. The report helps us narrow the field; it does not replace expert visual assessment.

Diamond proportions: why there is no single magic formula

You will often see narrow ‘ideal’ ranges quoted for table and depth percentages. These figures can be useful screening tools, but they should not be treated as universal pass-or-fail rules.

In a round brilliant, table size, total depth, crown angle, pavilion angle, star length, lower-half length, girdle thickness and culet size all interact. A proportion that works beautifully in one combination may be less successful in another. GIA’s research recognises multiple combinations capable of producing excellent brightness and fire.

For that reason, avoid buying a diamond from table and depth percentages alone. Use the complete grading report, quality imagery and an experienced assessment together.

Girdle thickness

The girdle is the narrow outer edge of the diamond. If it is extremely thin in vulnerable areas, the risk of damage may increase. An excessively thick girdle can retain weight where it contributes little to face-up size. Variations around the stone also matter, so the complete description should be considered rather than relying on a single preferred phrase.

Culet size

The culet is the point or small facet at the base of the pavilion. On many modern brilliant cuts it is graded None. A larger culet may be visible through the table and can influence the diamond’s pattern, although an open culet may also be an intentional feature of some antique-style cuts.

Polish and symmetry

Polish describes the quality of the facet surfaces. Symmetry considers the precision of the diamond’s shape and the alignment of its facets. They contribute to finish and are reported separately, but neither should be mistaken for a complete measure of light performance.

How to assess fancy-shaped diamond cut

Fancy shapes require more visual judgement because an overall GIA cut grade is not provided for them. Proportions are still useful, but personal preference plays a larger role.

Look for:

• an appealing outline and balanced length-to-width ratio
• an attractive, even pattern of light and dark
• good brightness across the stone
• minimal distracting windowing or large lifeless areas
• a bow-tie that does not dominate elongated shapes
• appropriate durability around points, corners and thin girdle areas
• a pleasing face-up size for the weight

What is the bow-tie effect?

Ovals, pears and marquises commonly show some bow-tie pattern through the centre. It is not simply a defect caused by ‘light leakage’, and expecting it to disappear completely is unrealistic. The question is whether the bow-tie adds attractive contrast and movement or appears dark, broad and distracting.

What is windowing?

Windowing is an area that appears transparent, allowing you to see through the stone more than intended. It can occur when facets do not return light effectively to the viewer. It is best assessed visually rather than inferred from one measurement.

Does cut affect how large a diamond looks?

Yes. Two diamonds of the same carat weight can have different measurements and face-up appearances. A deep diamond may conceal more weight vertically and look smaller from above. A very shallow stone may spread wider but sacrifice pattern, durability or brightness.

The aim is not simply to find the largest diameter. It is to find a diamond with pleasing spread and beautiful light performance.

Does a better cut cost more?

Cut quality influences price, but there is no reliable universal percentage difference between grades. Price also changes with carat weight, colour, clarity, fluorescence, shape, laboratory report, market conditions and the individual character of the diamond.

If your budget is fixed, it can be sensible to protect cut quality and adjust a factor that is less visible to you – perhaps moving slightly in colour or selecting an eye-clean clarity grade. The right balance depends on the shape, setting and your own preferences.

How to read the grading report

For a natural round brilliant, review:

• report number and issuing laboratory
• measurements and carat weight
• colour and clarity grades
• cut, polish and symmetry grades
• table and depth percentages
• crown and pavilion information, where shown
• girdle and culet descriptions
• fluorescence
• clarity plot and comments, where included
• laser inscription information, if applicable

Verify a GIA report through GIA Report Check using the report number. Remember that a report describes and grades the loose diamond at the time of examination; it is not a valuation, guarantee or proof of current ownership.

Should you consider light-performance imaging?

Additional tools can add useful information. The AGS Ideal Report by GIA, where available for an eligible diamond, assesses light performance including brightness, fire and contrast from centre to edge. Other imaging systems may illustrate patterns of light return or leakage.

These tools can help compare diamonds, but they should complement – not replace – the grading report and a real visual assessment. Ask what the image measures, how it was captured and how the result should be interpreted.

Common diamond cut mistakes

Choosing size at the expense of beauty

A bigger number on paper is not always a bigger visual impression. Compare face-up dimensions and light performance alongside carat weight.

Treating one proportion chart as a guarantee

Proportions interact. A narrow online range can exclude beautiful diamonds or include stones you may not love.

Assuming all fancy shapes with Excellent polish and symmetry are excellently cut

Polish and symmetry are finish grades, not an overall fancy-shape cut grade. You still need to assess outline, pattern, bow-tie, windowing and brightness.

Looking only under jewellery-store spotlights

Ask to view the diamond under diffused indoor light, near natural daylight and in lower light. A beautiful diamond should have character beyond one carefully directed spotlight.

Choosing your diamond at Xennox

The best diamond cut is the one that combines sound craftsmanship with the kind of beauty you respond to. Some people love crisp brightness; others prefer broader flashes, softer antique character or the elegant contrast of a step cut.

At our Brisbane studio, our gemmologists and diamond specialists help you compare the details without losing sight of the feeling. We consider the grading report, proportions, light performance, visual spread and the way the diamond will work within your ring design.

Xennox has been creating jewellery in Brisbane since 1976. Our rings are made by hand in our workshop, allowing the diamond and the setting to be considered together from the beginning.

Book a diamond consultation in Brisbane to compare carefully selected diamonds side by side and discover which one feels unmistakably yours.

Frequently asked questions

What is the best diamond cut grade?

For a natural standard round brilliant, GIA Excellent is the highest cut grade and a sensible place to begin. The final choice should still be made by comparing the diamond’s proportions, pattern and appearance.

Does GIA grade the cut of oval diamonds?

GIA does not currently provide an overall cut grade for oval diamonds. It reports measurements, polish and symmetry, among other characteristics, so visual assessment remains essential.

Is diamond shape the same as diamond cut?

No. Shape is the outline, such as round, oval or pear. Cut quality is the craftsmanship and proportioning that influence how the diamond handles light.

Can a smaller diamond look better than a larger one?

Absolutely. A beautifully cut diamond can look brighter, livelier and sometimes larger face-up than a heavier diamond that hides weight or returns light poorly.

Is a grading report enough to choose a diamond?

No. It is an essential independent description, but it cannot fully communicate personality, pattern or what your eye prefers. Use it with expert guidance and visual comparison.