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Diamond Proportions Guide: Why it Matters More Than Weight

Diamond weight, which is measured in carats, does not tell the full story. When it comes to diamonds, many people think that the bigger (weightier) the diamond, the better. However, this is not always the case. In fact, diamond proportions are usually more important than diamond weight.

This means that a diamond smaller in weight can be just as beautiful, valuable, and optically big as a diamond of larger carat size. This is because quality and cut play a large role in the final diamond's appearance. In this blog post, we will discuss why diamond proportions matter more than carat weight.

Cut Is the Most Important of the 4Cs

When it comes to diamonds, cut is the most important factor to consider. Even though cut is often used interchangeably with 'shape’, they are different. This is because the cut determines how well the diamond reflects light and sparkles. A well-cut diamond will have more brilliance and fire than a poorly-cut diamond of the same weight and shape. For round brilliant diamonds — the only shape with an official cut grading from a lab (IGI/GIA) — the cut grades are: Ideal (for IGI only), Excellent, Very Good, Good, Fair, and Poor (for GIA only). Each grade will affect the light performance and appearance of the stone's brilliance.

In addition to the cut, other factors can affect a diamond's sparkle. These include clarity, colour, symmetry, and polish. However, the cut has the most significant impact on sparkle from all of these factors. This is why proportions matter more than weight; they directly affect how well the diamond will reflect light and its lustre.

Cut grade is determined mainly by the stone's proportions, which are essentially the specific measurements of the diamond that affect how it sparkles. It is important to note here that even two diamonds of the same carat weight can have different face dimensions, which affects light performance and optical size. For example, a diamond's table size (the large flat facet on the top of the stone) affects how light enters the diamond and is reflected back to the viewer's eye. A small table size will cause light to leak out of the side of the stone, while a large table size will make the diamond appear dark.

When a diamond is cut to proper proportions, light is able to enter the stone and reflect back out, creating the stunning sparkle that we all know and love. However, if a diamond is not cut to proper proportions, the light will enter the stone but will not reflect back out correctly. This results in a dull, lifeless diamond. This is often referred to as the difference between a diamond with moderate depth and one that is too shallow or too deep.

Diagram of how light travels through diamonds

Besides being more beautiful, diamonds that are cut to proper proportions are also more valuable. This is because these diamonds are rarer than those not cut correctly. Thus, if you are looking for a high-quality, valuable lab grown diamond, be sure to pay attention to the diamond's proportions.

How Diamonds Are Cut and Polished

To further understand why a diamond's proportions matter more than weight, it's helpful to have a basic understanding of how diamonds are cut and polished. After a lab grown diamond is formed, it's cut into a variety of different shapes.

The cutting process is done with a rotating wheel that is covered in abrasive grit. The diamond is slowly shaped into its finished form, which can take anywhere from hours to days. The most popular shapes include round, marquise, pear, oval, emerald, princess and asscher. If the diamond is not cut with precision, its facets may not reflect light evenly, resulting in a duller overall appearance.

Cut and polished diamonds

After the cutting process is complete, the diamond is ready to be polished. This is where a skilled diamond cutter uses precise instruments to create symmetry and polish the diamond's surface until it shines.

The polishing process can have a major impact on a diamond's appearance. For instance, if the diamond is not evenly polished, it may appear darker in some areas and lighter in others. Finally, the diamond is inspected for quality and then sent off to be made into fine jewellery.

Diagram of diamond proportions

Choosing the Ideal Diamond Proportions

Now let's discuss how to choose a diamond with ideal proportions. First, it is crucial to understand the different parts of a diamond. The four main parts of a diamond are the crown, pavilion, girdle, and culet.

The crown is the upper part of the diamond, while the pavilion is the lower part. The girdle is the narrow band that goes around the middle of the diamond, and the culet is the small point at the bottom of the pavilion.

When choosing a diamond's cut, you should look for one with a well-proportioned depth % and table % in your IGI/GIA grading report. For a round cut diamond, the total depth percentage is ideally 59 to 62% (the diamond depth divided by the diamond's diameter), and the table percentage is ideally 53 to 60% (the length of the table divided by the diamond's diameter).

The width of the girdle is also important – avoid anything with the extremely thin or extremely thick designation. As the ideal or excellent diamond proportion changes based on the particular diamond shape, check out a diamond proportion guide below. When looking at your certificate, ensure your diamond sits in the ideal to excellent range:

The following percentages contribute to an excellent cut stone.

Lab Grown Diamond Shape Depth Percentage Table Percentage
Pear 57.5 to 65 55.5 to 63
Oval 58 to 63 56 to 62
Marquise 57.5 to 64.5 56 to 64
Princess 65 to 73 65 to 74
Emerald 60 to 68 58 to 68
Cushion 62 to 68 55 to 62
Heart 51.9 to 61 58 to 64
Round 56 to 63 52.5 to 63.5

As well as paying attention to the depth % and table % for diamond proportions, you should also ensure that the diamond has a small culet. A small culet ensures that light will enter and exit the diamond correctly, resulting in maximum brilliance and fire. The culet should be graded as "none" or "pointed" on your GIA/IGI grading report.

Now you know why diamond proportions matter more than carat weight and how to choose the ideal diamond proportions. This will assist you in finding the perfect lab grown diamond for your needs. For ideas, get inspired by Cullen Jewellery’s latest lab grown engagement ring collection today!


A diamond's light performance is one of the most important factors in determining its beauty and value. The way a diamond interacts with light affects everything from its colour to its clarity.

When light enters a round diamond, it is reflected off the surface and then refracted back through the diamond. This creates a dazzling display of brilliance and fire. The angle at which light passes through the diamond also affects its appearance.

A well-cut diamond will reflect light evenly, creating a uniform sparkle. In contrast, a poorly cut diamond can appear dull and lifeless. The light performance of a diamond is just as important as its physical characteristics, and it is something that should be taken into consideration when shopping for a new stone.

This is why cut grades (officially from a lab only for a round diamond) and a diamond's depth matter in the IGI/GIA certificate. For example, the ideal depth percentage for a round brilliant cut diamond will differ from another diamond's cut and shape. For the same price, you may be able to find a diamond with superior brilliance if it's cut effectively.

The better diamond is sometimes the one with the better table width or width ratio that prevents the diamond from having a dark appearance or undesirable qualities. A diamond's beauty is subjective, but aspects like a thick girdle, a slightly thick culet, star length, pavilion depth, and the table facet can affect how the diamond size appears.

You can have two diamonds of similar carat size but drastically different depth proportions. The table above recommends the best diamond proportions for various fancy shapes and cut grades such as for a round diamond, though other elements like crown angles and pavilion depth may affect your decision.

A diamond's shape and a diamond's facets can affect its light performance. Indeed, you can evaluate the diamond table, pavilion depth, cut grade, girdle thickness, star length, crown height, and diamond depth among other characteristics before simply looking at the carat weight of a cut diamond.

While diamond price plays a role, length to width and cut quality and their interplay with the best proportions should matter, too. Round diamonds are the most popular, and that's why the diamond proportions should be compared with the table above.

A round brilliant diamond may have, for instance, a different pavilion angle, table size, or girdle thickness than another, and that's why jewellers pay attention to different factors during an evaluation, such as crown angles or crown height. A diamond proportions chart helps in this regard, where you can compare round-cut diamonds and other cut diamonds for uniformity.

A diamond's depth affects the ideal diamond proportions as well. Factors like the table facet, which is the largest facet of a diamond, should be considered alongside the diamond's size. Avoid diamonds that have been cut poorly, especially if you can find a similar diamond that reflects light upward and simply presents better. Ultimately, when it comes to light performance, a diamond's brilliance is crucial and will be impacted by cut quality.

The ideal proportions for the crown and pavilion are based on the width of the girdle, which should be 3-3.5% of the total depth of the diamond. Girdle thickness is the width of a gemstone's girdle, which is the band that separates the crown from the pavilion. Generally speaking, the crown proportion compared to the diamond's total depth, also known as the crown height percentage, should be 12.5-17%.

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