What Are the Proportions of the Ideal Diamond Cut?

The Meaning of Ideal Cut

When referring to round-cut diamonds, a cut is considered ideal if its proportions allow the stone to retain and reflect as much light as possible back to the eye of the observer, maximizing the brilliance and sparkle of the diamond.

Although there are different standards for ideal cut, the proportions they specify do not differ significantly.

On the other hand, the further a diamond’s proportions are from the measurements that are considered ideal, the more of the light that enters the stone leaks out of it.

Since the facets of poorly cut diamonds are not well aligned to retain maximum light, such stones appear dull and lifeless.

Different Ideal Cut Standards

Although it may seem that the definition of ideal cut is uniform, there is actually no universal agreement as to what the exact ideal proportions should be.

In reality, there are at least half a dozen standards for the measurements of the ideal cut.

The most widely used benchmarks today are the American Ideal Cut, the Practical Fine Cut, and the Scandinavian Standard.

The first one is used in North America, and the other two are mainly used in Europe.

Key Diamond Cut Dimensions and Proportions

One of the most accurate ways to determine the quality of a diamond’s cut is to examine some of its key measurements and calculate proportions.

But before you start checking diamond dimensions, you should be familiar with several terms regularly mentioned when measuring and evaluating diamond cut:

Girdle: This is the outer edge of the stone, and most diamond proportions are expressed as percentages of the girdle’s diameter (for round stones) or width (for non-round shapes).

The girdle itself can be graded on its thickness, and cuts with extremely thin or extremely thick girdles are undesirable; anything between these two extremes is acceptable, with the ideal girdle being thin to slightly thick.

Culet: This is the facet at the very bottom of the diamond. A stone can have no culet at all, and this is also acceptable.

The ideal culet is very small to small, with medium and big culets being less desirable.

Table percentage: The table is the topmost facet of the stone, and table percentage is calculated by dividing the table diameter or width by the diameter or width of the girdle.

Depth percentage: This is the distance from the table to the bottom of the stone divided by the girdle diameter or width.

Round Cut Ideal Proportions

Although there are different standards for ideal round cut, the proportions they use as guidelines are more or less similar.

The first thing you should know about the ideal measurements is that they are expressed as a percentage of the diameter of the diamond’s girdle, which is simply the stone’s edge and is also its widest part.

Let’s look at the most important measurements for an ideal-cut diamond. The numbers are given as ranges that include the standards of the most widely used benchmarks:

Table diameter: 53%-57.5% (This is the diameter of the diamond’s flat top, i.e. the table.)

Crown height: 14.4%-16.2% (This is the vertical distance from the girdle to the top of the stone; the crown is the upper part of the diamond, from the girdle to the table.)

Pavilion depth: 43.1%-43.2% (This is the vertical distance from the girdle to the bottom of the diamond; the pavilion is the lower part of the diamond, from the girdle to the lowest point.)

So, using the above numbers to derive a rule-of-thumb benchmark, we can conclude that an ideal-cut diamond should have a top that has a width of 53% to 57.5% of the girdle, a pavilion that is around 43.1%-43.2%, and a crown that is around 14.4%-16.2% of the girdle diameter.

The total height of an ideal diamond, measured from its flat top to the stone’s pointed bottom, will be around 57.5%-59.4%.

If you are shopping for a diamond, you most likely won’t need to measure all these values yourself: They will be included in the diamond’s grading report, and you should look them up. It is not recommended to buy diamonds that do not come with such a certificate that verifies their quality.

What about other diamond cuts?

Unlike the classic round cut, most other cuts do not have as detailed, precise, and widely accepted benchmarks against which to measure cut quality. There are guidelines, but they are much looser, and they vary from cut to cut.

What’s more, many grading labs do not grade the cut of non-round diamonds. The best way to evaluate the cut of such stones is to compare them and see which one has more brilliance as well as which diamond looks better (or even bigger) when worn.

Below you can see the ideal proportions suggested for the most popular non-round diamond cuts.

Oval Cut, Marquise Cut, Pear Cut, and Heart Cut Ideal Proportions

The recommended dimensions for the oval cut also largely apply to the cuts that are derived from it: the marquise cut, the pear cut, and the heart cut.

For all these cuts, the ideal table percentage is in the 52%-64% range. A value as low as 51% or as high as 68% is also acceptable and is a sign of a good cut. Cuts with a table percentage of 50% and lower or 69% and higher are not considered top quality.

The recommended depth percentage for the oval cut and its derivatives is between 58% and 62%, with values as low as 53% and up to 70% also considered acceptable. Depth percentages lower than 53% and higher than 70% are not recommended.

The length-to-width ratios recommended for these cuts are the following:

• Oval: Between 1.25 and 1.60; ideally in the 1.30-1.50 range.
• Marquise: Between 1.65 and 2.30; ideally in the 1.85-2.00 range.
• Pear shaped: Between 1.35 and 1.80; ideally between 1.40 and 1.55.
• Heart shaped: Between 0.80-1.10; ideally between 0.90 and 1.05.

Princess Cut Ideal Proportions

Ideally, the table percentage of a princess cut should be in the 60%-75% range. It is not recommended to buy a princess cut whose table percentage is higher than 80% or lower than 56%.

The ideal depth percentage for this cut is between 65% and 75%. Values outside of the 60%-80% range are not considered desirable.

Princess cuts with a length-to-width ratio in the 1.00-1.05 range are the most desirable – i.e., such stones look square, whereas higher ratios result in a more oblong look.

Cushion Cut Ideal Proportions

The ideal table percentage for cushion cuts falls anywhere between 58% and 70%. Values higher than 71% and lower than 56% are not desirable.

The recommended values for the depth percentage of cushion cuts are similar: ideally, in the 58%-70% range; lower than 56% and higher than 71% – not recommended.

The optimal length-to-width ratio for non-square cushion-cut diamonds is between 1.10 and 1.30.

Radiant Cut Ideal Proportions

The table percentage of an excellent or very good radiant cut should be in the 60%-70% range. Values that fall between 54% and 74% are also acceptable, while table percentages beyond this range are not recommended.

The ideal depth percentage for a radiant cut is also in the 60%-70% range. Values below 57% and above 74% are not desirable.

The most sought-after length-to-width ratio for non-square radiants is in the 1.10-1.40 range.

Emerald Cut and Asscher Cut Ideal Proportions

The emerald cut and the Asscher cut are basically the same – the latter is just the square version of the former.

A table percentage in the range of 60%-70% is considered best for these cuts, and values as low as 55% and as high as 73% are still considered good. Avoid Asscher and emerald cuts whose table percentage is lower than 54% and higher than 74%.

Depth percentage in these cuts should ideally be between 60% and 70%. Values lower than 57% and above 74% are not recommended.

For emerald cuts, the most preferred length-to-width ratios fall in the 1.20-1.80 range.

The ideal length-to-width ratio for Asscher cuts is between 1.00 and 1.05.

The Diamond Cut Grading Scale: A Shortcut to Evaluation

You don’t necessarily need to examine the above-mentioned measurements in detail for each stone you are looking at.

Luckily, the GIA (Gemological Institute of America) does the job for you, and by taking into account a variety of standards for lengths and angles, it assigns a cut grade to each diamond it certifies.

If the diamond you are considering comes with a GIA grading report, take a look at it.

The GIA grades diamond cut using the following categories: Excellent, Very Good, Good, Fair and Poor.

In general, anything that is graded Good and higher is a fine option. Avoid diamonds graded Poor; those with a Fair cut are of so-so quality, so it’s up to you to decide whether such a stone is worth it.

We recommend that you only buy diamonds that are graded Excellent, as cut is the one dimension that you shouldn’t skimp on – after all, it determines how brilliant the diamond is.

Click here to see a selection of diamonds with ‘Excellent’ cut and their prices.

In case a stone doesn’t come with a certificate, you can assess its cut using the proportion guidelines outlined above (for round cuts or other cuts). However, we do not recommend that you buy diamonds that do not come with a grading report.

Using cut grades to determine quality works best for round diamonds. Other cuts don’t have such well-defined grading standards, but you can use the key diamond ratios that were mentioned earlier as a quality guide.

How Diamond Cut Works

Cut affects three major characteristics of a diamond that define how light interacts with the stone: brilliance, dispersion and scintillation.

Brilliance is seen when light entering the stone is reflected back to the eye of the observer. If a diamond’s depth (or height) is too high or too low, the light going into the diamond will not be reflected back to create brilliance but will instead leak out of the stone, remaining unseen.

As a result, the brilliance of too deep or too shallow diamonds is poor.

Dispersion is when the structure of the diamond breaks down the entering light into a spectrum of colors. The result is visible as play of colors when looking at the diamond and is also called “fire”.

Scintillation is the ability of a diamond to sparkle in intermittent flashes as you move it before your eyes. A well-cut stone exhibits a lot of sparkle, whereas a poorly cut one looks much less lively.

How Important Is It to Buy an Ideal-Cut Diamond?

The closer the cut of a round diamond is to the ideal cut, the higher the stone’s brilliance will be.

Diamonds with poor cuts look less bright and appear lifeless, not to mention that because of the diminished brightness, their flaws and color tints become more visible.

So, it is usually a good idea to pay more money for an Ideal/Excellent cut if you can afford it. In any case, don’t go lower than one grade down from Excellent/Ideal – i.e. don’t buy a cut that’s rated lower than Very Good unless you are fine with the diminished brilliance that comes with the price savings.