Buying a Round Cut Diamond: Everything You Should Know

There’s a reason the Round Cut diamond shape is so popular, accounting for over 66% of all diamonds purchased. That reason can be found in physics — the way that light interacts with this most beautiful of all diamond cuts.

What Exactly Is a Round Cut Diamond?

It’s a cone-shaped diamond with 58 facets. The fastest way to understand it quickly is to look at an example.

You can see 3D rotatable examples of them at James Allen.

The Round Brilliant Cut is cut in such a way as to maximize brilliance, based on how light behaves within a clear diamond gemstone.

What is brilliance? It’s the name we give to the sparkling fire of light that shoots and pours from the top of a well cut diamond.

In more objective terms, that would be the amount of light entering the diamond from any direction which is refracted and reflected back out of the top of the diamond.

You wouldn’t want the light to be reflected out through the sides of the diamond. Nor would you want it to exit from beneath the diamond. You want it to exit from the top of the diamond, since that is the direction from which the diamond is most naturally viewed.

That’s the objective explanation of why a diamond is judged beautiful by most people. It all depends on its brilliance. Brilliance is defined objectively by how much light is reflected through the top of a diamond.

The Round Brilliant Cut shape of diamond is the most efficient, most effective architecture of diamond to cause light entering the diamond to reflected back out of the top of the diamond.

When Did People First Catch on to the Brilliant Properties Round Cut Diamonds?

It was in the beginning of the 20th Century, in 1919, when Marcel Tolkowsky published “Diamond Design: A Study of the Reflection and Refraction of Light in Diamonds.”

In his work, he described why the Round Brilliant Cut shape of diamond is objectively and subjectively best for maximizing brilliance. It’s even better than the most popular diamond cut of the 19th Century, which was known as the Old European Cut of diamond.

The Old European Cut emphasized the color of a given diamond. The Round Brilliant Cut emphasizes brilliance — that efficient refraction and reflection of light for which diamonds are the most desired gemstone.

As more and more people laid eyes on the fireworks shows that can result from Round Brilliant Cut shape diamonds, this cut took off in popularity.

The popularity hasn’t died down. It’s unlikely to, since based on the physics of light and of diamond, there’s no better way to cut a diamond to maximize its brilliance.

If a diamond is forever, then so is the Round Brilliant Cut.

What to Look for in a Round Brilliant Cut Diamond

Beauty is said to be in the eye of the beholder, and the eyes of almost everyone beholding Brilliant Round Cut diamonds admire the following characteristics.

They look for …

  • Classic appearance: This cut has been around since the 19th Century, and gained preeminence during the 20th Century, shortly after WWI. Incredibly brilliant, and with a classic shape, it’s a statement of gorgeous style and taste which is always on point.
  • Beauty: Beauty is often tied to symmetry, and a Round Brilliant Cut diamond is symmetrical when viewed from any right angle. The reflection and refraction which results in their brilliance and beauty only serve to accentuate the perfectly symmetrical shape.
  • Style: The Round Brilliant Cut shape of diamond doesn’t clash with anyone’s style, from the most casual to the most elegant. Like many timeless looks, it complements anyone and anything they wear
  • Brilliance: With its 58 facets, the Round Brilliant Cut gathers light from all sides and sends most of that light out through the top of the diamond. This is the one of the two main qualities for which diamonds are prized, and the Round Brilliant Cut is superior here. Brilliant is quite literally its middle name.
  • Fire: Fire technically differs from brilliance in that Brilliance refers to all light, while Fire refers to a subset of light which is the rainbow color effect from refraction within the diamond. Lovely flashes of multicolored light stream from the top of a Round Brilliant Cut in a volume that is unmatched by any other cut.

Not Every Diamond Is Equal

Light doesn’t change. Diamond stone is essentially or very similar in refraction and reflection of light, from stone to stone, within a certain range of clarity at least.

Since diamond cutters, together with help or research from physicists, have worked out the dimensions to maximize fire and brilliance from a diamond via the Round Brilliant Cut shape, you might suppose that all are created equal.

They’re not equal, and here’s why. They vary from diamond to diamond in the following ways:

Difference #1: Cut Quality

Even though there might exist a Platonic ideal, mathematically ideal cut of Round Brilliant Cut, not every diamond can be cut in that way. Inclusions (flaws) in the diamond may prevent a diamond cutter from cutting along a certain plane. The size of a diamond, relative to the sizes of diamond popular in the market, may prevent certain cuts. The skill of the diamond cutter is also important. If you make a mistake in cutting a diamond, you can’t redo it. Finally, no diamond has perfectly platonic ideal inner structure. So a plane of cleavage might not be perfectly straight.

Cut has five different grades in a GIA diamond grading report:

Excellent Virtually all the light exits through the table. Maximum fire and brilliance.
Very Good Most of the light exits through the table. Most people who aren’t trained for grading diamonds can’t tell much difference between Very Good and Excellent.
Good Much of the light exits through the table. These diamonds are still beautiful. They give a lot of pleasure. And they can be a good value because they’re not as expensive as Very Good or Excellent.
Fair Much of the light exits through the pavilion and culet of the diamond, depending on the shape). Not great fire and brilliance.
Poor Almost all the light exits through the pavilion and culet. Not a great experience at all. Very minimal fire and brilliance.

Difference #2: Clarity

As you may know by now in your diamond research and shopping, the Clarity of a given gemstone of diamond varies by the individual stone. Clarity is classed into several grades by the GIA.

The GIA Diamond Clarity Scale has 6 main categories. Some of these categories have grades subdivided inside of them. So there is a total of 11 specific grades.

  • Flawless (FL) No inclusions and no blemishes visible under 10x magnification
  • Internally Flawless (IF) No inclusions visible under 10x magnification
  • Very, Very Slightly Included (VVS1 and VVS2) Inclusions so slight they are difficult for a skilled grader to see under 10x magnification
  • Very Slightly Included (VS1 and VS2) Inclusions are observed with effort under 10x magnification, but can be characterized as minor
  • Slightly Included (SI1 and SI2) Inclusions are noticeable under 10x magnification
  • Included (I1, I2, and I3) Inclusions are obvious under 10x magnification which may affect transparency and brilliance

A diamond with a Clarity grade of Included or Slightly Included will have less brilliance and fire than one of Internally Flawless or Flawless. So, even though Cut has a greater influence on fire and brilliance than does Clarity, it’s obvious that Clarity plays its part. It’s one factor that goes into an individual diamond’s brilliance.

Difference #3: Color

Color affects Round Brilliant Cut diamonds less dramatically than it does other cuts. Round Brilliant Cut diamonds create such brilliance and fire that it covers up any problems with color — to an extent.

(In the case of white diamonds, another name for colorless diamonds, which include most of the diamonds on the market, “color” is bad. It refers to the yellowish tint of color which makes white diamonds less desirable and less valuable.)

The amount of light coming through the top of a Round Brilliant Cut diamond is so great that it hides weaknesses in color, makes them difficult to perceive.

Color is still noted in the diamond grading report from the GIA. Color also still does affect the price, even if, to the naked eye, any yellow tint is not distinguishable from the top in natural light.

Finally, depending on the amount of color, it does at last begin to affect how a diamond looks. 

The GIA color grade begins with D, which perfectly colorless, and ends with Z, which is well into the yellowish, amber colored diamonds of industrial grade. (D is the first grade in the scale. There are no A, B, or C in the GIA color scale.) Most jewelry stores don’t sell diamonds of more than L color.

A Round Brilliant Cut diamond can look colorless to everyone except a highly trained professional even in some cases up to J and K color — especially when paired with a yellow gold setting.

Difference #4: Carat Weight & Size

If everything else is about equal, then a larger Round Brilliant Cut diamond creates a larger effect on the eyes than a smaller one.

Difference #5: Proportion

Depending on the qualities of the diamond and the skill of the diamond cutter, the Round Brilliant Cut diamonds may differ.

Imagine a vector 3D object on a computer screen.

You can pull corners of it out and in. You can make it short and squat or tall and thin. You can deform it.

In a less radical way of course, diamond cutters have to first imagine the gemstone they want to create from a given rough diamond.

Can it be the perfectly brilliant proportion every single time? Not even close. Inclusions prevent it. Mistakes prevent it. Maybe even style and taste prevent it in some cases.

The best proportioned Round Brilliant Cut diamonds result in the most fire and brilliance, by definition. These proportions are known. The relations are complex. It’s unlikely a non-expert could master these with any skill. It’s quite difficult even for trained gemologists, since:

  1. The results (brilliance and fire) epend on physics of diamonds and light
  2. The variables, even within a single well defined 58-facet shape such as Round Brilliant Cut, are literally infinitely variable. Two examples will suffice: What is the length of the diamond as measured from top to bottom? What is the angle of the pavilion facets to the table?

Proportion matters very much. But it’s difficult or impossible to master. So it makes sense to rely on a diamond grading report from the GIA. Look for the Cut and Clarity and Color grades.

Look most of all for as much fire and brilliance as you can find. Notice when a diamond appears dull. Notice when one stands out for its brilliance.

As you develop more and more experience looking at Round Brilliant Cut diamonds, you’ll begin to see each one of them as an individual. Your taste and insight will grow.

You Have the Final Choice When It Comes to Your Diamond

While objective grading reports from professional experts are invaluable in the retail diamond market, the final authority on what you like is you.

Take into consideration what the experts say about any diamond, in its diamond grading report, also called a certificate. (Never buy a diamond of any expense without a grading report from the GIA or AGS.)

But also take into account what you like about any given diamond.

You can sift through an enormous number of Round Brilliant Cut diamonds from reputable retailers online, such as James Allen, Blue Nile, or Brian Gavin.

Notice what you like and don’t like. With the information we’ve provided, judge for yourself of clarity, color, cut, proportion.

When You’re Shopping for ANY Diamond

  1. Look first for a high Cut grade in the GIA report — Excellent or Very Good.
  2. Then look among diamonds of J and K Color, if you’re on a budget.
  3. Next, look among diamonds of S1 and S2 (Slightly Included) Clarity.
  4. Finally, look at Carat weight, and dial it up until you reach the limit you’ve set aside in your budget.
  5. You’ll get the best Round Brilliant Cut diamond possible, for the money.

Experiment with the filters at these stores as you search.

The old way of shopping for diamonds, which was looking at one after another in a store, is gone forever, for many people.

It’s so much easier to search with exactly your parameters of budget, Cut, Clarity, Color, and Carats, and also shape, using the filters at online retailers with extremely high quality, stunning images of individual diamonds.

Try the filters and save functions at these three online retailers, to begin your search and to begin learning how to choose what will become your Round Brilliant Cut diamond:

  • James Allen
  • Blue Nile
  • Brian Gavin