Because carat weight is the ubiquitous size measurement for retail diamond information, it’s the measurement shoppers use.  This can be a source of confusion, especially for shoppers comparing diamonds of different qualities and shapes because Carat is a diamond’s weight, not its size.  There are reasons for why it’s used though, and below we’ll delve into them as well as the confusion it can create and how to still make an informed decision.

Why is it used?

Because it’s easier, and under certain circumstances, it’s good enough.

Let’s start with the part about it being easier:

While a variety of factors go into a diamond’s price, pricing is measured per carat in the diamond industry.  A diamond’s shape and quality characteristics will dictate the price per carat, but in the end it’s the standard unit for pricing used so it’s what suppliers and retailers are used to.

It’s not just that though…

A diamond is a 3-dimensional object, as such, its size is comprised of length, width, and depth.  For a round diamond, in which the target shape around the girdle is a circle, you would measure its diameter instead of length and width, but even exceptionally cut round diamonds aren’t perfect circles, so they are listed with a diameter range.  Regardless of a diamond’s shape you are looking at keeping track of 3 numbers to get a true sense of its size.  These numbers are small and exact, measured in millimeters down to the hundredths place.

If you were presented with two diamond measurements, 6.60mm x 6.70mm x 3.89mm and 6.50mm x 6.54mm x 4.08mm, it would be hard to identify which is larger.  If you were instead given 1.01-carats and 1.08-carats, it’d be easy to identify the larger of the two even without having certainty on what a carat is.

How can it be good enough then?

The reason it can be good enough is because there are standard proportions diamond cutters strive for and under certain circumstances a diamond’s weight can be highly correlated to its measurements. 

Those circumstances are very specific though, and they are:

• The diamond’s shape must be round.
• The diamond must be very well cut.

Both circumstances must be met for there to be a strong correlation between a diamond’s carat weight and its actual dimensions.  The Gemological Institute of America (GIA), the standard-bearing independent gemological lab in the world, only issues a cut grade for round diamonds (click here to learn more about their grading and why we only use GIA reports on our diamonds).  This is because round is the only shape where there are ideal proportions, consistently recognized industry wide, and because only very well-cut diamonds are certain to be close to this ideal proportion. 

When a round diamond is graded by any respected gemological lab, the considerations used to grade its cut include assessing:

• The diamond’s weight and depth relative to its diameter at the girdle.
• The depth of its girdle.
• The angle of its crown and pavilion.
• The size of its table

There are even more minute assessments made but these are the ones that clearly effect the overall shape of a round diamond.  For each of these assessments there are tolerable ranges that equate to a grade, and the ranges are smallest at the highest grades. 

So when you have 2 very well cut round diamonds, you can confidently use carat weight to compare their sizes because the range of all these proportions are very close to each other, as well as to the ideal.

Click here to see our Diamond Quality guide and see how little variation there is amongst Excellent Cut diamonds relative to lower cut grades.

What about round diamonds that aren’t very well-cut? Or other diamond shapes?

Poorly cut round diamonds:

The lower a round diamond’s cut grade, the greater the range of all the aforementioned proportions will be.  Not only will this diminish the overall look and brilliance of the diamond, but it can mean that in the above example of one diamond being 1.01-carats and another being 1.08-carats, the 1.01-carat diamond actually looks bigger as it is the diamond with the larger diameter. 

This is because many of the considerations for the quality of a round diamonds cut are related to the diameter at its girdle, which is widest part of the stone between the crown (top) and the pavilion (bottom).  When you look at a diamond in a ring or other piece of jewelry, you tend to assess its size by the diameter at its girdle, because you are looking at it from the top down, with its depth largely obscured by the piece it’s within.  That 1.01-carat diamond may have a shallow crown or pavilion angle (or both) leading to a shallow overall depth and a disproportionately large diameter, whereas the 1.08-carat may have too much depth and an undersized diameter.  This creates a circumstance where set in a ring the 1.01-carat diamond looks visibly larger than the 1.08-carat.  There are innumerable combinations that can pop up comparing poorly cut diamonds because there are so many ways in which they can deviate from the ideal proportions, but in all instances those diamonds will have less brilliance and correlation between their carat weight and visual size than very well-cut diamonds will have.

Other diamond shapes:

While only round diamonds are reliably given a cut grade, that doesn’t mean that any other shape diamond, commonly referred to as fancy cuts, are poorly cut.  A round diamond is meant to be a circle at its girdle, and there isn’t much room for interpretation when it comes to what a circle is.  Anything past a small amount of deviation is now an oval, which itself is a common diamond shape.  But is there one kind of oval?  That’s the issue with assessing a fancy cut diamond.  You can have a long and thin oval, or a short and wide oval, and you can have the same when it comes to a cushion cut or a pear-shape.  For each of them, many will find one proportion more appealing than the other, and vice versa.  The inherent shapes are less defined, so the GIA only gives fancy cuts Polish and Symmetry grades, which are subsets of an overall Cut grade.  They assess the remaining, less subjective parts of how a diamond is cut.

Because other shapes are less rigidly defined than circles, the correlation between carat weight and visual size for a fancy cut diamond is much looser.  This means that you could have 2 diamonds that are the same type of shape and carat weight, and they’d look starkly different.  It also means that two diamonds of different shapes, but the same carat weight may look totally different.  In each scenario, using carat weight as the only measurement for size can lead to getting a stone that looks drastically different than your expectation, and this can have further adverse effects such as changing the overall look and construction of your dream ring.  If your chosen ring isn’t already designed to account for this, that can mean detracting from the ring’s look and durability.

Why That Specific still uses Carat weight:

Despite the pitfalls laid out above when it comes to referring to diamonds by their carat weight, that is still how we list diamonds.  There are a few reasons for this:

• We only source diamonds that receive an Excellent cut grade or better, so the correlation between carat weight and size is very high, and there’s no risk of comparing a well-cut diamond with a poorly cut one.
• Currently we only source round diamonds, so there’s no risk of different shapes being compared.
• Our rings are designed to proportionally adapt to any diamond between 0.30-carats and 3.00-carats so regardless of the size, our rings will be perfectly proportioned to fit any diamond regardless how you measure it.
• While we use carat weight as the initial size descriptor, we clearly list the diamonds actual dimensions once we’ve sourced your specific diamond, so you’ll know exactly what you’re getting.