Diamond Cut

Diamond Education › Cut

Diamond Cut — The Most Important of the 4Cs

Of all the factors that determine a diamond's beauty, cut is the most important. It's also the most misunderstood. Cut does not refer to a diamond's shape — it refers to how well the diamond has been crafted from the rough stone. A diamond's cut determines how it interacts with light, and light is everything.

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Why Cut Matters More Than Anything Else

A diamond that is cut too shallow will allow light to escape through the bottom. A diamond cut too deep will lose light through the sides. Only a well-proportioned diamond — one cut to precise mathematical angles and percentages — will return light to your eye as brilliance, fire, and scintillation.

This is why a beautifully cut G colour, VS2 clarity diamond can outperform a D colour, FL clarity stone that is poorly cut. Cut is the great equaliser — and the most important decision you'll make when choosing a diamond.

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The Three Elements of Diamond Light Performance

Brilliance

The white light reflected from the interior and surface of a diamond. A brilliant diamond appears bright and lively. Brilliance is the result of a diamond's proportions — table size, crown angle, pavilion depth, and culet size all play a role.

Fire

The dispersion of light into the colours of the spectrum — the rainbow flashes you see when a diamond moves in light. Fire is influenced by the crown angle and the facet arrangement of the stone.

Scintillation

The pattern of light and dark areas, and the flashes of light that appear when a diamond, the light source, or the observer moves. A well-cut diamond has a balanced, attractive scintillation pattern.

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Diamond Anatomy

To understand cut, it helps to know the parts of a diamond. Each named region plays a role in how light travels through the stone.

Table — the large flat facet at the top of the diamond. Its size is expressed as a percentage of the girdle diameter.

Crown — the upper portion above the girdle. The crown angle controls how much fire (dispersion) the diamond shows.

Girdle — the narrow band around the widest part of the diamond. The girdle separates the crown from the pavilion.

Pavilion — the lower cone-shaped portion below the girdle. The pavilion angle has the greatest effect on brightness.

Culet — the very point at the bottom (or a tiny flat facet, if present). In a well-cut diamond the culet is None or Very Small.

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GIA Cut Grades

The Gemological Institute of America (GIA) grades cut quality for round brilliant diamonds on the following scale:

Cut Grade	Description
Excellent	Maximum brilliance and fire. The highest grade. Reflects nearly all light that enters. Recommended.
Very Good	Reflects nearly as much light as Excellent. Excellent value.
Good	Reflects most light. A good balance of cut quality and value.
Fair	Reflects some light but noticeably less than higher grades.
Poor	Loses significant light. Generally not recommended.

At Diamond Imports, we focus primarily on Excellent and Very Good cut grades. Life is too short for a dull diamond.

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GIA Excellent Proportions for Round Brilliant Diamonds

For a round brilliant to receive GIA's Excellent cut grade, its measurements need to fall within a specific range. The values below are drawn from GIA's published research on diamond cut grading.

Proportion	Excellent Range
Table size	52% to 62%
Total depth	57.5% to 63%
Crown angle	31.5° to 36.5°
Pavilion angle	40.6° to 41.8°
Star length	~50% (typical)
Lower-girdle facet length	~75% (typical)
Girdle thickness	Thin to Slightly Thick
Culet size	None to Very Small

GIA uses a deductive grading system: the diamond's overall cut grade is determined by the lowest grade among any individual parameter. A diamond can be Excellent on every measurement except one — and that one weaker parameter will pull the overall grade down. This is why all parameters matter, not just table and depth.

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What About Fancy Shapes?

This is one of the most important — and least discussed — facts in diamond buying:

This is not a flaw in the GIA system — it's an honest admission. Fancy shape diamonds have such varied facet patterns and light-performance characteristics that GIA does not believe a single objective grade can fairly describe them all. Two oval diamonds with identical colour, clarity, and carat weight can perform very differently in real life, and the certificate alone cannot tell you which is which.

What this means for buyers: for any fancy shape, the gemologist's trained eye is more valuable than the certificate. The shape needs to be seen, turned, and assessed in person. This is exactly what we do at Diamond Imports — every fancy shape diamond is hand-selected and inspected before it ever reaches our inventory.

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Cut vs Shape

It's worth clarifying: cut and shape are different things. Shape refers to the outline of the diamond — round, oval, cushion, emerald, pear, and so on. Cut refers to the quality of the craftsmanship within that shape. A round brilliant diamond can be cut Excellently or Poorly. The same is true of every other shape.

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Polish and Symmetry

Polish refers to the smoothness of each facet's surface. A diamond with Excellent polish has no surface imperfections visible under 10x magnification that could affect light performance.

Symmetry refers to the precision of the facet arrangement and the alignment of the diamond's features. Excellent symmetry means every facet is in exactly the right place, maximising light return.

When a certificate shows Excellent/Excellent for Polish and Symmetry alongside an Excellent cut grade, this is sometimes called a Triple Excellent or 3EX diamond — the highest possible cut quality designation.

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