What is diamond cut?
When most people hear "diamond cut," they think of the shape — round, oval, princess, emerald. That is not what cut means in gemology. Cut refers to the quality of the cutting — the precision of the angles, the symmetry of the facets, the proportions of the stone's geometry.
A round brilliant diamond has 57 or 58 facets. Each one is placed at a specific angle. When those angles are right, light enters through the top, bounces around the inside, and exits back through the top in an explosion of white light and colour. When those angles are wrong — too deep, too shallow, off-centre — light leaks out the bottom or the sides. The diamond looks dull. Dead. Empty.
This is why cut is called the most important C. Colour and clarity are properties of nature — the diamond was born with them. Cut is a decision made by a human being with a polishing wheel. A great cut can make a lower-colour, lower-clarity stone look spectacular. A poor cut can make a flawless, colourless diamond look flat.
A GIA cut grade for standard round brilliant diamonds evaluates the overall face-up appearance of a polished diamond based on its proportions, symmetry, and polish. The grade predicts the brightness, fire, and scintillation a trained observer would see under standard viewing conditions. GIA introduced the cut grading scale in 2005 after fifteen years of research.
Why cut matters more than the other three Cs
Here is a simple test. Take two diamonds of identical colour (G) and identical clarity (VS1) and identical carat weight (1.00ct). One has an Excellent cut. The other has a Poor cut. Place them side by side under display lights.
The Excellent cut diamond blazes. It throws light across the room. You can see fire — flashes of spectral colour — from two metres away. The Poor cut diamond looks like glass. Flat, dead, lifeless. A customer who knows nothing about gemology will instinctively pick up the Excellent cut stone. The eye knows.
The difference in price between those two stones is significant. But the difference in appearance is exactly the reason people fall in love with diamonds in the first place. Cut is what makes a diamond a diamond — not just a piece of transparent mineral.
The proportions that determine cut quality
GIA evaluates cut quality by measuring seven key proportions. Each proportion affects how light behaves inside the stone. Here is what the cross-section of a round brilliant looks like, and what each measurement means.
Cross-section of a round brilliant diamond showing the key proportion measurements that determine cut quality
Table percentage
The table is the large flat facet on top. Table percentage is the table's width expressed as a percentage of the diamond's total diameter. The ideal range for a round brilliant is 53–58%. Too large a table reduces the crown's ability to disperse fire. Too small a table reduces overall brilliance.
Depth percentage
Total depth percentage is the diamond's height from table to culet, as a percentage of its average diameter. The ideal range is 59–62.5%. A stone too deep (above 64%) looks smaller than its carat weight suggests — its mass is hiding underneath. A stone too shallow (below 57%) leaks light straight through the pavilion.
Crown angle
The angle of the crown facets relative to the girdle plane. GIA identifies 34–35° as ideal for round brilliants. Crown angle controls fire — the dispersion of white light into spectral colours. It works in combination with pavilion angle; both must be right simultaneously.
Pavilion angle
The angle of the pavilion facets relative to the girdle plane. This is the single most critical proportion measurement. At 40.6–41°, the pavilion returns the maximum light back through the crown. Steeper angles create a dark centre. Shallower angles create a "fish-eye" — a reflection of the girdle visible through the table.
Girdle thickness
The girdle runs around the widest part of the diamond. It should be Thin to Slightly Thick. An extremely thin girdle risks chipping during setting. An extremely thick girdle hides weight without adding visible diameter — the diamond looks smaller than its carat weight.
Culet size
The culet is the tiny point at the very bottom. In modern round brilliants it should be None or Very Small. A larger culet, common in antique diamonds, appears as a dark circle in the centre when viewed face-up.
The GIA cut scale
GIA introduced its cut grading scale for standard round brilliant diamonds in 2005, after fifteen years of research modelling light behaviour across thousands of proportion combinations. Before GIA, no major laboratory had formally graded cut. The scale has five grades.
| Grade | What it means | Face-up appearance | Recommendation |
|---|---|---|---|
| Excellent | Proportions within optimal range. Maximum light return. | Brilliant, fiery, exceptional sparkle. Minimal dark areas. | Always aim for this. The benchmark. |
| Very Good | Slightly outside optimal but still high performance. | Very bright and attractive. Difference from Excellent is subtle. | Excellent value. Hard to distinguish from Excellent. |
| Good | Noticeably outside optimal in one or more proportions. | Attractive but lacks the pop of Excellent/VG. | Only if budget forces a choice. Better to buy smaller Excellent. |
| Fair | Significant departure from optimal. Considerable light leakage. | Noticeably less brilliant. May appear glassy or dark centre. | Not recommended for centre stones. |
| Poor | Proportions far outside any acceptable range. Severe leakage. | Dull, lifeless. The stone's beauty is largely wasted. | Avoid entirely. |
The three components of light performance
When a well-cut diamond is described as "alive," three separate optical phenomena are happening simultaneously. Understanding them helps you evaluate a diamond in person.
Brilliance
Brilliance is the white light returned to your eye when you look at the diamond face-up. The overall brightness and glow. It comes primarily from the pavilion facets bouncing light back up through the crown. A diamond with ideal pavilion angles (40.6–41°) shows intense, even brilliance across its entire face. Tilt it, and the brilliance shifts — the stone appears to breathe.
Fire
Fire is the dispersion of white light into its spectral colours — the flashes of red, orange, yellow, blue, and violet you see when the diamond or light source moves. It is caused by the crown facets acting as prisms, bending different wavelengths of light at different angles. Higher crown angles (up to about 35°) generally produce more fire. This is why, in direct sunlight, some diamonds appear to throw coloured sparks across the wall.
Scintillation
Scintillation is the dynamic sparkle pattern — alternating bright and dark areas — visible when the diamond, the observer, or the light source moves. A well-cut round brilliant held under a single overhead point light source shows a specific pattern: an eight-pointed star of bright facets in the table. Poor symmetry produces an uneven, patchy pattern. This is one reason Hearts and Arrows diamonds command a premium — their scintillation pattern is perfectly symmetrical and visually satisfying.
Cut quality vs diamond shape
This is the confusion that trips up most buyers. Shape and cut are two entirely separate things.
Shape is the diamond's outline when viewed from above — round, oval, pear, cushion, emerald, princess, marquise, asscher, radiant, heart. Shape is a stylistic decision. It says nothing about quality.
Cut quality is the precision of the workmanship within that shape. A round brilliant can be Excellent or Poor. An oval can be superbly proportioned or badly proportioned. The shape does not determine the quality — the cutter's skill does.
Among all shapes, the round brilliant is the most studied and most optimised for light performance. Mathematicians and gemologists have spent over a century working out the precise geometry that maximises light return from a circular stone. This is one reason round brilliants command a 20–40% price premium over fancy shapes of equivalent carat weight and quality — they are demonstrably more efficient at returning light, and cutting them wastes more of the rough stone.
Hearts and Arrows — the pinnacle of round brilliant cutting
The highest tier of round brilliant cutting produces a specific optical signature. Viewed through a special eyepiece called a Hearts and Arrows viewer (a type of idealscope with a red reflector), a perfectly cut stone shows eight perfectly symmetrical hearts visible in the pavilion and eight perfectly symmetrical arrows visible in the crown. This pattern only appears when every facet angle and alignment is within extremely tight tolerances — typically ±0.2° on crown and pavilion angles, and near-perfect facet-to-facet symmetry. Hearts and Arrows diamonds carry a 10–20% premium over standard Excellent cut diamonds.
How to choose — practical guidance
For a round brilliant, the guidance is clear: always buy Excellent. The premium over Very Good is typically 10–15%. It is worth paying. Never buy a larger stone at a lower cut grade. A 0.90ct Excellent cut diamond looks bigger face-up than a 1.10ct Good cut because better-cut stones spread their weight more efficiently across the diameter.
| Shape | Ideal depth % | Ideal table % | Length-to-width ratio |
|---|---|---|---|
| Round brilliant | 59–62.5% | 53–58% | 1.00 (circular) |
| Oval | 58–62% | 53–63% | 1.30–1.50 |
| Cushion | 61–67% | 61–67% | 1.00–1.05 or 1.15–1.35 |
| Princess | 64–75% | 67–72% | 1.00–1.05 |
| Emerald | 61–67% | 61–69% | 1.30–1.50 |
| Pear | 58–64% | 53–63% | 1.45–1.75 |
| Marquise | 58–62% | 53–63% | 1.75–2.25 |
| Asscher | 61–67% | 61–69% | 1.00–1.05 |
Diamond cut in the Indian context
India — and specifically Surat in Gujarat — cuts and polishes approximately 90% of the world's diamonds by number. The vast majority are smaller stones, under 0.30 carats, cut at high volumes across Surat's main diamond zones: Varachha, Katargam, Udhna, and Mahidharpura.
For these smaller stones — the trade calls them "melee" — GIA cut grades are not typically assigned. The cost of individual certification would exceed the stone's value. Buyers and traders assess cutting quality through loupe examination and proportion measurement. Computer-aided planning software from Sarine Technologies and OGI Systems has significantly raised the standard of Surat-cut commercial melee over the past two decades.
For larger stones (above approximately 0.30ct), Indian cutting factories increasingly produce GIA-certified Excellent cut diamonds. A stone planned in Surat, cut in a Surat factory, polished and inspected in Mumbai, and graded at GIA's Mumbai laboratory can be on a jeweller's bench in New York, London, or Dubai within weeks of leaving the mine.
Frequently asked questions
Is Excellent cut always better than Very Good?
By GIA's criteria, yes. But the visible difference between a top Excellent and a top Very Good is subtle in normal viewing conditions. If your budget forces a choice between a 1.00ct Very Good and a 0.90ct Excellent at the same price, choose the Excellent. The cut difference will be more apparent than the size difference.
Does cut grade affect price significantly?
Yes. Excellent commands a 10–20% premium over Very Good and 30–50% over Good for otherwise identical stones. The premium reflects both superior appearance and the additional rough material sacrificed to achieve ideal proportions — a more precisely cut stone generally wastes more of the rough.
Can a lab-grown diamond have an Excellent cut?
Yes, absolutely. Cut grade is determined by the geometry of the polished stone — it is entirely independent of whether the diamond was mined or grown in a reactor. Lab-grown diamonds are cut and polished using identical equipment and techniques, and graded by the same laboratories. A lab-grown Excellent cut and a natural Excellent cut with the same proportions behave identically optically.
What is a "super ideal" or "Hearts and Arrows" cut?
Super ideal is an informal trade term for round brilliants cut to the most precise tolerances, producing the Hearts and Arrows pattern under a viewer. They fall within GIA's Excellent grade but represent its very top end. Brands like Brian Gavin Signature, Whiteflash A-Cut-Above, and James Allen True Hearts specialise in them. They command a 10–20% premium and are primarily for buyers who prioritise optical performance above all else.
Diamond colour — the D-to-Z scale explained · Diamond clarity — inclusions and what they mean · Carat weight — size vs weight explained · GIA — how the world's top grading lab works