Cut is one of the most important characteristics of a diamond, as its proportions determine how light is reflected by the stone. To determine the quality of a certain cut, you can check how it is graded, look at the stone’s brilliance, or measure the diamond’s dimensions.
Checking Cut Grades
One of the easiest ways to check the quality of a diamond’s cut is to look at its grade, provided that the stone comes with a grading certificate.
One of the most respected grading labs, the GIA (Gemological Institute of America), evaluates the cut of round diamonds by assigning the following grades: Excellent, Very Good, Good, Fair, and Poor.
Other labs have different scales and grade names, but they all follow the same logic: The closer the dimensions of a certain cut are to the set of proportions considered ideal, the higher the grade assigned.
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 some key diamond ratios as a quality guide.
Checking the Brilliance of Cuts
Brilliant cuts, such as the round cut and the princess cut, are designed to maximize the sparkle of the diamond, and that’s why you can use this characteristic to determine how well cut a stone is.
You can compare two diamonds of the same cut, and if one has more brilliance than the other, it is very likely that the more brilliant stone has a higher-quality cut.
This method is applicable only when comparing brilliant cuts, though.
If you are evaluating step cuts, such as the emerald cut and the Asscher cut, which are not made to enhance brilliance, then it’s best to look at the diamond’s dimensions and proportions.
Checking Diamond Cut Dimensions and Proportions
One of the most accurate ways to determine the quality of a stone’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 Proportions
The ideal table ratio for a round cut is in the 53%-58% range. Round cuts graded Very Good usually have tables in the 52%-53% and 58%-60% ranges. Values below 51% or above 64% are characteristic of fair or poor cuts.
The total depth ratio should ideally be in the 57.5%-59.5% range, but values as high as 64% are still considered good.
The depth of the pavilion (the part below the girdle) should ideally be around 43% of the girdle diameter, but values as low as 41% and as high as 45% are also acceptable. Values outside the 41%-45% range are not considered desirable.
Oval Cut, Marquise Cut, Pear Cut, and Heart Cut 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 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 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 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 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.