Evaluating Diamond Size

The Accuracy Of Face-up Size Evaluation

This page will try to explain how diamond size evaluation works on this site. In short: a diamond face-up area is compared to a face-up area of a reference diamond, which is calculated differently for each diamond shape. Based on a deviation from reference diamond area, an evaluation of whether or not a diamond is of adequate face-up size is made.

How accurate is this evaluation? It depends on a diamond shape. It's accurate for Round cuts and approximately accurate for fancy shapes. The system was calibrated and tested against a database of 700,000 real world diamonds. Details are explained below.

Why Is Face-up Size Important?

Diamond's face-up size
Face-up size is what you see when a diamond is mounted in a setting. It's important for a diamond to look its weight, otherwise you're paying for weight you can't see. In most cases, inadequate face-up size also means a duller, less brilliant, less beautiful stone.

Diamonds are sold by weight, which means diamond cutters, in order to maximize their profits, try to retain as much weight from the rough as possible. If a diamond is cut to ideal proportions, it will exhibit the most brilliance and sparkle, however, this means that the yield from the rough will be lower (translation: lower carat weight, less profit for the cutter). Therefore, diamonds tend to be cut in a way to retain as much weight from the rough, sacrificing beauty for weight.

One way to determine if a diamond is cut properly is to check its face-up size. Two diamonds of the same shape can be exactly the same weight, but one can look bigger than the other.

For example, 1 carat Round should have a diameter of about 6.5 mm. If a diamond is cut too deep, its diameter might be 6.1 mm or even lower. But it's not only smaller size that is the problem, the diamond will also have non-optimal light performance, i.e. – it will be less brilliant and sparkly, less beautiful that an ideal cut stone. The same thing happens, if a diamond is cut too shallow (e.g. 1ct Round with 6.7 mm diameter).

Face-up size is a crucial diamond characteristic and one should always make sure that a stone looks its weight. Here is how this site evaluates face-up size:

Evaluating Round Cuts

Round diamonds are compared to a reference Round diamond with the following proportions:
Proportions:
Table: 57%
Crown angle: 34°
Pavilion angle: 40.7°
Girdle: 2.8%
Star length: 50%
Lower half-length: 80%
Culet: None
Calculated values:
Depth: 60.3%
Crown height: 14.5%
Pavilion height: 43%
The process: Knowing reference diamond proportions, one can calculate a diameter for any given weight and then the face-up area, which is the area of the girdle plane.

When you enter a diamond of arbitrary weight and size, the application calculates the face-up area of the entered diamond and compares it to the area of the reference diamond of the same carat weight. Diamonds can be evaluated as Adequate, Borderline, or Inadequate:

Adequate
Diamonds with face-up area of within 5% lower and 3% higher than reference diamond area are considered to be of adequate face-up size.
Borderline
Diamonds with face-up area of 5-7% lower or 3-5% higher than reference diamond area are considered to be borderline.
Inadequate
Diamonds with face-up area lower than 7% or higher than 5% than reference diamond area are considered to be of inadequate face-up size.

Example: 1 carat Round reference diamond measures 6.5 mm in diameter and has a face-up area of 33.18 mm². Any 1ct Round with an average diameter between 6.34 mm and 6.6 mm (or a face-up area between 31.52 and 34.18 mm²) would be considered adequate.


Evaluating Non-Round Diamonds - Fancy shapes

Non-round diamonds or fancy shapes are not so easy to compare, because of their unique geometry. Since every diamond is unique in terms of outline, it's impossible to accurately calculate face-up area given only length and width. Face-up area of fancy shapes is always an estimation (except for Princess cuts).

For fancy shapes, approximation formulas are used to calculate the size of the reference diamond and then estimate the face-up area. Area estimation is based on empirical analysis. I've tried to come up with geometrical formulas but they were more inconsistent then the empirical approach.

Here's what I did: I looked for photos of diamonds and then calculated the percentage difference between the area of a rectangle obtained by multiplying the length and the width of a diamond and the actual surface area of the diamond's outline. This was done with Photoshop by counting pixels in the photos. This method is actually very accurate.

For each diamond shape, I analyzed around 30-50 photos. I looked for normally shaped diamonds in terms of outline as well as the extremes. This gave me a range of area differences and based on this analysis I was able to make an informed decision about the area adjustment factor that I will use for a particular shape.

To indicate the precision of these approximations, a plus or minus (±) sign is stated next to the face-up area of fancy shapes (e.g. ±5%). I would say that the vast majority of real world diamonds would fall into these ranges.

Because of all this, the evaluation of fancy shapes is not as strict as with rounds. It depends on a shape, but for a diamond to be considered adequate its estimated area would usually need to be within 9% compared to reference diamond area (9-12% is borderline, greater than 12% is inadequate).


Conclusion

Face-up size is an important factor to consider when choosing a diamond, as one probably wants a sparkly and beautiful stone with as close to ideal proportions as possible. While the evaluation of the face-up size offered by this site is not exactly an exact science, it can definitely help you determine whether or not a diamond looks its weight.

For Round cuts, you can pretty much rely on it. For fancy shapes, it's not as accurate, but if nothing else it can help you sift out the really poor cuts, keeping you from making bad purchasing decisions.

I would generally advise against buying diamonds evaluated as inadequate. Borderline stones can sometimes still be a good choice, but that's your call.

The bottom line: A diamond must look its weight. Make sure it does.