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Understanding Diamond Proportion (Ratio)

by Willyou.net * Sep 18, 2021
Emerald cut diamond - Understanding Diamond Proportion (Ratio)

The way that a rough diamond is cut is by far the most significant factor behind its sparkle. Yes, it’s simple enough to learn the different shapes diamonds are cut into – and which cuts sparkle the most, and which ones you prefer – but, if you stop there, you’ll be overlooking the most important factor of all: proportion.

What do we mean by that? Well, two emerald cut diamonds can look totally different based on their proportions – despite the fact that they are both cut according to the same shape.

The same holds for round diamonds and fancy cut diamonds – even an eye clean, colorless diamond can be totally derailed by poor proportions, and appear pretty lifeless alongside one with ideal proportions.

Yes, it’s another complication, but one that you’ve got to go face-to-face with if you want to get the best possible diamond for your budget.

What Does Diamond Proportion Mean?

The ratios that exist between the diamond’s table, depth, and width, after it has been cut into the appropriate shape. These are represented in percentages, and used to determine the strength of a diamond’s cut, its value.

One of the biggest challenges diamond cutters face is determining how to create the biggest possible diamonds from an irregularly shaped rough stone – while also avoiding any major inclusions contained within it and, of course, ensuring good proportions.

A diamond’s value increases significantly with size, but getting the largest possible diamond out of the rough stone would typically mean creating a diamond with some very strange proportions. It may be far too deep, with a very long pavilion, or it may be too shallow, with a table percentage way too high to give good sparkle.

Either way, the best approach is often the one that balances size with proportion. The biggest possible proportioned diamond will prove most popular among those looking to create beautiful diamond jewelry.

How Important are Diamond Proportions?

Very important. In fact, probably the most important thing to search for, alongside eye cleanliness.

Diamonds with excellent proportions will deliver better light performance – or, more specifically, better brilliance, fire, and scintillation. It is all too easy for a diamond to be cut too shallow or deep and, in both cases, this will mean that less light is able to take the optimal pathway through the diamond, and out of the table.

As a shopper, understanding the way each diamond’s proportions will impact its visual appeal is essential, through it’s a little more complicated than memorizing the color and clarity grades. You’ve got to know what percentages to look for, and which ones to avoid entirely.

What are Good Proportions for a Diamond?

A GIA graded diamond with ‘Excellent’ or ‘Very Good’ proportions should be your focus during your search. The ‘right’ proportions depend entirely on the shape of the diamond, since what works perfectly for a Round cut diamond will look pretty bizarre on an Asscher or Emerald cut diamond.

In a nutshell, good proportions will mean that a diamond is neither too shallow nor too deep, and neither too wide nor too narrow. Its structure will support its beauty, rather than obstructing it.

These ideal proportions will comprise depth and table percentages, the culet (lowest point), the girdle (where the crown meets the pavilion), and height to length ratio. Below, we will list the ideal cut diamond proportions for each diamond but, for now, we will look in closer detail at the ideal diamond cut proportions for the Round cut diamond, and for the Emerald cut diamond – two totally different shapes – to give you a good idea of the different proportions required to maximize sparkle.

The Round Cut Diamond

 ExcellentVery Good
Length to Width Ratio1.00 – 1.0091.01 – 1.019
  Table  56% – 59.9%54% – 55.9% Or 60% – 60.9%
  Depth  59% – 62.3%585 – 58.9% Or 62.4% – 63.5%
CuletNoneVery Small
GirdleThin to Slightly ThickVery Thin to Slightly Thick

The Emerald Cut Diamond

 ExcellentVery Good
  Length to Width Ratio  1.35 – 1.441.30 – 1.34 Or 1.45 – 1.50
  Table  60% – 65.9%58% – 59.9% Or 66% – 72.9%
  Depth  63% – 67.9%59% – 62.9% Or 68% – 69.9%
CuletNoneVery Small
GirdleVery Thin to Slightly ThickVery Thin to Thick

Here, you can see that the table of an Emerald diamond with ideal cut proportions will cover more of the diamond’s width than the table of a Round diamond, also with ideal cut proportions.

Similarly, a Round cut diamond graded as ‘Excellent’ will need to be the same all the way around, necessitating a length-width ratio of 1 (or within a hundredth of a decimal of 1). For an Emerald cut, the long, slender, shallow shape  will mean that a ratio of 1 is far from ideal.

This is why, for square diamonds like the Asscher cut, an ‘Excellent’ length-width ratio is also one that is as close to 1 as possible.

Text Box: INSERT IMAGE: diamond profile viewSome factors, such as culet size, remain the same across the board but, for the most part, proportion is entirely dependent on the shape and cut of the stone.

How Important is Diamond Table?

The table is an intrinsic part of any diamond. Its job is to allow light into the diamond, to disperse it between the diamond’s many facets, and to reflect it back from the pavilion and out the top of the diamond as brilliance and fire.

If you’re feeling like the diamond table is playing two totally different roles, you’d be right. It’s got to let in light, but also refract light onto the crown, rather than letting it straight back out the table. It’s got to be small enough to do that, but not so small that the diamond looks dull and sparkly.

As a result, getting the proportions of the diamond table right is absolutely vital to creating a good diamond – and it’s not easy. Thankfully, your job is a lot easier, and simply involves getting to know the ‘ideal’ proportions for diamond tables – and knowing the trouble that can be caused by one that is out of proportion, even if it’s just a small amount.

What Does the Girdle Do?

It’s there to offer some protection to the diamond, particularly during the process of setting it within jewelry.

Like the culet, a sharp point can be more prone to breaking. As a result, both the culet and the girdle may be thickened to ensure more protection to the stone during its transferal into the ring, and throughout its lifetime on the wearer’s finger. 

A thicker girdle does not cause as much disruption to the diamond’s appearance as a culet but, even so, thinner girdles are generally considered the preferable option.

What Does it Mean When a Diamond is Spread?

A spread diamond is another term for shallow cut diamonds.

Shallow cut diamonds are, in essence, diamonds that do not feature the ideal diamond cut proportions. Yes, Emerald cut diamonds are shallower than round cut diamonds – a big selling point for people who want to get a bigger surface area for a more affordable carat size – but, like any cut, it is possible to go too shallow.

The Ideal Cut Diamond Proportions

Below, we have put together the ideal proportions (graded ‘Excellent’ by the GIA) for each diamond cut, so that you can approach your chosen shape with the right information at your side.

Remember, however, that an experienced jeweler will be incredibly well-versed in this subject, and won’t steer you in the wrong direction. They understand the importance sparkle holds for engagement rings better than anyone, so don’t panic about learning these numbers off by heart.

 Length to Width RatioTableDepthCuletGirdle
Asscher1.00 – 1.02960% – 65.9%63% – 67.9%NoneVery Thin
to Slightly Thick
Cushion1.00 – 1.02960% – 65.9%63% – 67.9%NoneVery Thin
to Slightly Thick
Cushion (Rectangle)1.18 – 1.2860% – 66.9%63% – 67.9%NoneVery Thin
or Slightly Thick
Emerald1.35 – 1.4460% – 65.9%63% – 67.9%NoneVery Thin to Slightly Thick
Marquise1.90 – 2.0053% – 65.9%58% – 62.9%NoneVery Thin
to Slightly Thick
Oval1.35 – 1.4255% – 62.9%58% – 62.9%NoneVery Thin
to Slightly Thick
Pear1.55 – 1.6253% – 62.9%58% – 62.9%NoneVery Thin
to Slightly Thick
Princess1.00 – 1.02967% – 72.9%  67% – 72.9%NoneVery Thin
or Slightly Thick
Radiant (Square)1.00 – 1.02961% – 69.9%61% – 67.9%NoneVery Thin
or Slightly Thick
Radiant (Rectangle)1.22 – 1.3260% – 66.9%63% – 67.9%NoneVery Thin
or Slightly Thick
Round1.00 – 1.00956% – 59.9%59% – 62.3%NoneThin to Slightly Thick

So, Does Diamond Proportion Matter?

Yes, it matters a great deal. The wrong dimensions can undermine a diamond’s light performance, and turn an otherwise beautiful stone into something not worth your time.

Again, as we’ve said, it’s not like you’ve got to get these tables memorized. Your jeweler isn’t going to ask you to recite them like you’re back in kindergarten.

Even so, it’s massively helpful for you to go into the process understanding the basics of diamond proportion, and why it is so important to anyone looking to buy the best possible diamond for their bride to be.