One of the 4 Cs of diamond grading, cut affects a diamond’s brilliance, fire and scintillation. Diamond Cuts are graded for the quality of cut: the proportions, symmetry and polish. Some of our most popular diamond cuts include the round brilliant cut, princess cut, emerald cut, marquise cut and cushion cut. Whichever diamond shape you require for your diamond rings, engagement rings, diamond earrings or other jewellery, read our essential guide here.
One of the all-important 4 Cs, cut describes the shape and facet arrangement of a diamond as well as the quality of the faceted stone.
A diamond’s cut describes its shape and facet arrangement. The most popular diamond cut is the round brilliant cut. However, many different facet arrangements and shapes can be achieved due to the introduction of laser cutting. Formerly, diamonds could only be sawn in certain directions due to their differential hardness. However, lasers can cut a diamond crystal in any direction. This has resulted in a whole range of fancy-cut diamonds.
For more information on any of the diamond cuts themselves, click the below:
- Round Brilliant Cut
- Princess Cut
- Emerald Cut
- Asscher Cut
- Pear Cut
- Oval Cut
- Marquise Cut
- Cushion Cut
- Baguette Cut
- Trillion Cut
- Old Cut
- Rose Cut
- Fancy Shapes
Quality of Cut
The quality of cut is dependent upon three key factors: the proportions of the cut, the symmetry of the cut and the polish. The GIA published their ‘Diamond Cut Grading System’ in 2004 following 15 years of research, and the main criteria for this is outlined below.
The proportion aspect of the cut criteria refers to the angles between facets and can be split into the following areas:
- Girdle Diameter
The diamond’s outermost edge is called its girdle. The diameter of the girdle is used as a basis for proportion dimensions. The girdle diameter is classed as 100%, and the various measurements in relation to the girdle diameter are calculated from this.
- Table Size
The table facet is the largest facet on the front of the diamond. The table measurement is calculated as a percentage of the girdle diameter. For example, in a well-cut round brilliant-cut diamond, the table size should be between 54 and 66% of the girdle diameter.
In a fancy cut stone such as pear or emerald cuts, the table percentage is taken according to the width of the table relative to the girdle width – instead of using the length measurements.
- Culet Size
The diamond culet is the point where the pavilion facets meet on the reverse of the stone. Perhaps surprisingly, the size of a diamond’s culet is assessed by looking through the stone’s table. Historically, diamonds have been cut with large culets, but modern diamonds have culets that vary in size.
It is important that a diamond has a culet. If there is no culet present on a diamond, damage is likely to happen to this point. On the other end of the spectrum, if the culet facet is cut too large, it acts as a ‘window,’ allowing light to escape through the back of the stone.
In a fancy-cut stone such as the emerald or pear cut, the culet is elongated, and is known as a ‘keel.’ The keel is assessed along its width in relation to the width of the girdle diameter.
- Girdle Thickness
If a diamond’s girdle is too thick, it will affect the way that light bounces around the stone. It will also retain weight that won’t necessarily be noticeable face up, making the stone appear small in relation to its carat weight.
However if the girdle is too thin, the durability of the stone is compromised. Girdle is measured on a scale from knife-edge to extremely thick. A girdle measurement of thin to medium is recommended to avoid the problems on either end of the scale.
- Crown Angle
The crown angle of a faceted diamond affects the crown facets and also alters the table size. Therefore, it is highly important in the appearance of the diamond face up. The crown angle can affect the balance of fire and brilliance seen in the stone. Brilliance is defined as the brightness resulting from the reflection of light from the diamond, and fire is best described as the breaking up of white light into spectral colours when it passes through the diamond.
The ideal crown angle for a round brilliant cut diamond is between 30 and 35°.
If the crown angle is too shallow, this makes the table larger and therefore sacrifices an amount of fire in the stone, whilst showing a greater brilliance. If the crown angle is too steep, the table is smaller, and there is less brilliance with more fire.
A diamond with ideal proportions offers the perfect balance of both fire and brilliance.
- Pavilion Angle and Depth
The pavilion angle of a round brilliant-cut diamond should be 41-42° to ensure the maximum return of light. This can be fairly easily assessed when viewing the diamond side on, and imagining half of a right angle, where the pavilion should extend from the girdle.
Equally, the pavilion depth of a round brilliant cut diamond should be between 43 and 45% of the girdle diameter. If the pavilion of the stone is too deep or shallow, the light can leak out of the back facets. This reduces the brilliance of the diamond, so the ideal pavilion angle is crucial.
If the pavilion depth is too shallow, the reflection of the girdle can be seen through the table of the stone. This optical effect is known as a ‘fish eye,’ named after the appearance of a grey ring in the table facet.
If the pavilion is too large, the diamond loses an amount of brilliance. The light goes through the stone and escapes through the back facets, giving the effect of a dark circle under the table of the stone. This effect is known as a ‘nail head
The symmetry of the cut of a stone is fairly obvious to discern. Whilst it may be hard to pinpoint the exact symmetry flaw, the cut will look unusual. Symmetry faults are split into major and minor faults for the purposes of diamond grading, and the range goes from poor to excellent.
Major Symmetry Faults:
- An off-round appearance, or an even outline for fancy-cut diamonds.
- If the table facet is off-centre.
- If the culet is off-centre.
- If the table and girdle are not parallel.
- A wavy girdle.
Minor symmetry Faults:
- When facets do not join properly.
- A misalignment of crown and pavilion facets, when viewed from the side.
- Any misshapen facets.
- A misshapen table.
- An extra facet.
- Naturals – A natural is an area of unpolished crystal left on the stone, often seen on the girdle.
The grading system is as follows:
No Symmetry Flaws (assessed with a microscope)
No Visible Symmetry Flaws (when viewed under 10x magnification)
Only Minor Symmetry Faults Seen
Visible Symmetry Deviations
Major Symmetry Faults
The polish of a diamond is essential for showing the maximum amount of brilliance, scintillation and dispersion of light. The quality of a diamond’s polish is assessed under 10x magnification. The presence of polishing lines and burn marks can affect the appearance of the polish.
Polishing lines appear as parallel lines on the stone’s surface, and burn marks appear as a white haze along facets. Polish is assessed from excellent to poor.
The finish of the girdle also is part of the cut. Diamond girdles fall into three types, bruted, faceted and polished.