The buyers' guide to round brilliant cut diamonds

Round brilliant cut diamonds are renowned for their unparalleled sparkle, making them the most popular cut used in diamond engagement rings today. Developed by Marcel Tolkowsky in 1919, the proportions, angles, and alignment of the round brilliant's facets are designed to optimise the reflection and refraction of light.

In this comprehensive buyers' guide, we set out the pros and cons of round brilliant cut diamonds and our expert recommendations for how to choose one. We also explain the anatomy of this iconic diamond cut, the history of how it was developed, and the most popular settings for showing round brilliant cut diamonds at their best.

Issues to consider before choosing a diamond

There are four main issues to consider before getting into the details of comparing different diamonds:

Budget

Start your diamond search by setting a budget that you can afford, then find your ideal diamond within your budget.

Personal priorities

Unless you have an unlimited budget, choosing the perfect diamond comes down to making sure you prioritise those things that matter to you most and possibly compromising on some things that matter to you less.

You might prioritise getting the largest diamond (largest carat weight), in which case you may need to compromise on quality (colour and clarity, but avoid compromising on the cut). Conversely, you might choose prioritising quality and accepting that you’ll get a smaller diamond.

Our recommendations will show you how to take a balanced approach: you don’t need to choose the very top colour and clarity grades to have a fantastic-looking diamond, but avoid compromising so much that you end up with a diamond that disappoints.

Natural (mined) vs. lab grown diamonds

You’ll need to decide whether you want a natural (mined) diamond or a lab-grown diamond. Again, this is a matter of personal preference. Mined diamonds are much more expensive than lab grown diamonds, but they are likely to hold their value better, and some people feel that a natural diamond is more romantic. Lab grown diamonds, on the other hand, will allow you to get a bigger and better quality diamond for your budget, as well as being a more sustainable choice.

Ethical sourcing

If you choose a natural diamond, ensure that it is traceable to its country of origin so that you can be confident it has not contributed to human exploitation or conflict. Unfortunately, despite what the diamond industry will tell you, Kimberley certification does not guarantee that a diamond is conflict-free or ethically produced. Be aware that Russia produces approximately ⅓ of all mined diamonds, and these are all Kimberley certified as being conflict-free despite helping to fund the Russian invasion of Ukraine. All our natural diamonds come from Canada, and those of 0.3ct and up all come with a CanadaMark certificate of origin. Ask your jeweller to provide proof of diamond origin.

Why choose a round brilliant cut diamond?

The round brilliant cut is the most popular shape of diamond for good reasons, but like any choice, it has both advantages and disadvantages when compared with other diamond shapes. To help you decide whether a round diamond is the best choice for you, we've set out their pros and cons below.

How to choose a round diamond

For those who have decided a round brilliant is the best cut for them, we've taken our experience of helping thousands of customers find their perfect diamond and boiled our expertise down to produce the set of recommendations in the table below.

If you want to understand more about our recommendations, we expand on them later. However, in order to be able to explain our recommendations, you first need to know about the structure of round brilliant cut diamonds and the terminology used to describe and assess them.

The anatomy of the round brilliant cut

Round brilliant cut diamonds typically have 57 facets (the flat surfaces cut into the diamond), though some have 58 if the culet (the tip at the bottom) is faceted rather than pointed. The facets are typically arranged as set out below:

• Table: the flat top surface of the diamond, this is the largest facet and plays a critical role in reflecting light.
• Girdle: The widest point of the diamond, forming a band around its circumference, which can be smooth, faceted, or slightly rough.
• Crown: the section of the diamond above the girdle, comprised of the table plus the following facets that disperse light into its component colours.
  • Eight star facets
  • Eight bezel or kite facets
  • Sixteen upper girdle facets (sometimes referred to as upper half facets)
• Pavilion: the lower portion of the diamond below the girdle, it is composed of 24 facets (excluding the culet) which reflect light back out through the crown.
  • Sixteen lower girdle facets (sometimes referred to as lower half facets)
  • Eight pavilion main facets
• Culet: a facet at the bottom tip of the pavilion, although some diamonds are designed with no culet, coming to a point instead).

The angles, proportions, and alignment of these facets determine the light performance of the diamond and are assessed when a diamond is given a cut grade.

The importance of diamond certificates when buying a diamond

A diamond certificate is vital because it sets out all of the key criteria you need to be able to compare one diamond with another.

However, you need to know that you can rely upon the information on the certificate, so it is equally important that you always buy a diamond with a certificate from a reputable independent grading lab like the GIA or IGI.

If you choose an uncertified diamond and rely instead on information from the jeweller who is trying to sell the stone, or you choose a diamond with a certificate from a less reputable lab, you run the risk that the quality of the diamond may be exaggerated in some way.

In the example GIA report above, you can see that the first section (top left) confirms the shape and cutting style of the stone ("Round Brilliant") as well as its dimensions, given as the range from the minimum to maximum diameter (in this case 6.57 to 6.60mm) and the total depth of the stone (in this case 4.12mm).

The next section, headed Grading Results, sets out the most important of the selection criteria, known collectively as the Four Cs:

• Carat Weight: This is the weight of the diamond, with one carat equal to 0.2g.
• Colour: Diamonds are graded for colour on a scale from D (colourless) to Z (light yellow or brown)
• Clarity: The clarity grade is an assessment of the inclusions or blemishes within the diamond.
• Cut: This is perhaps the most important consideration for round brilliant diamonds. The cut determines how effectively light entering the diamond will be refracted within and reflected back out of the stone. The GIA grades cut quality as Excellent, Very Good, Good, Fair, and Poor. For maximum brilliance, aim for a cut graded Excellent or Very Good. The IGI has a top cut grade of Ideal, followed by Excellent, Very Good and Good.

The third section, Additional Grading Information, provides the polish, symmetry, and fluorescence grades, as well as comments, such as any additional laser inscription on the stone or in the case of the certificate above, a comment about the fact that the Clarity Characteristics chart to the right does not show the positions of any pinpoint inclusions in the stone.

The fourth section (top right) is headed Proportions and provides a side-view diagram of the diamond annotated with all the measurements that have been taken in order to determine the diamond's cut grade. These include the table %, depth %, crown angle, pavilion angle, culet size, girdle size, star facet % and lower half %, and we will expand on these later.

The final section is headed Clarity Characteristics and is not present in all diamond certificates. It shows plots of the diamond from above and below and marks the positions and types of any inclusions within the diamond.

Key considerations when comparing round diamonds

By following our recommendations below, you will be able to make an informed decision and choose a stunning round brilliant cut diamond that offers both beauty and value.

All of the 'data' you need can be found on the diamond certificate, but it can also be helpful and reassuring to view diamonds you are considering, either in person or online via high-definition macro video and photographs.

Carat weight

Depending on your budget and preference, choose a carat weight that meets your needs but keep in mind that prices step up quite sharply at certain milestone weights, such as 0.65ct, 1ct, 1.5ct, and 2ct, for example.

Colour grade

The intense sparkle of round brilliant cut diamonds does a very good job of "hiding" any colour in the crystal itself. A grade of G is usually a good balance between cost and appearance. You might accept a slightly lower grade if you want a bigger stone for your budget, but be aware that the larger the stone, the more noticeable any colour will be. If you want a stone that looks colourless, you need to choose F or higher.

Clarity grade

While Flawless diamonds sound romantic, they are also the most expensive, and you don't need one for a beautiful stone. In fact, for a diamond of less than 1.5ct, a VS2 clarity stone is normally "eye-clean", meaning that it looks flawless to the naked eye. With larger diamonds, as the facets increase in size, it becomes easier to see inclusions within the diamond, so you may wish to go for a higher grade. We recommend at least VS1 for a stone of 1.5ct and above if you want it to be eye-clean.

Cut grade

Since the intense sparkle of the round brilliant cut is one of its major attractions, it doesn't make sense to compromise on this. Furthermore, as cutting techniques have improved over time, the premium you pay for a well-cut stone has declined. This is why we recommend aiming for the top cut grade: Excellent for a GIA-certified diamond or Ideal for an IGI-certified diamond.

Polish and symmetry grades

While having less impact on the overall attractiveness of a round diamond than the cut grade, we recommend aiming for the top polish any symmetry grades: ideally Excellent, though you are unlikely to notice any difference with the naked eye if you choose Very Good for either Polish or Symmetry.

Fluorescence

Some diamonds glow under UV light, which is called fluorescence. While usually not a problem in normal lighting conditions, strong fluorescence can sometimes make a diamond look hazy. For this reason, we recommend either choosing either no fluorescence or faint fluorescence.

Diamond proportions

These include all the various measurements and calculations that determine the overall cut grade. If you select a diamond with a GIA cut grade of Excellent or IGI cut grade of Ideal, then you don't really need to worry about the individual measurements below.

However, for those who like to get into the details, here are our recommendations:

• Table %
  • the diameter of the table facet, expressed as a % of the diameter of the girdle
  • 53%-58.5% gives the optimal balance between a large table to maximise the light entering the stone vs. larger crown facets to maximise the dispersion of that light
• Depth %
  • the depth (or height) of the diamond, expressed as a % of the diameter of the girdle
  • 59%-62.5% is optimal for maximising the light reflected by the pavilion of the diamond (minimising light leakage)
• Crown angle
  • the angle between the girdle and the crown facets
  • 34%-35% is optimal for light dispersion
• Pavilion angle
  • the angle between the girdle and the pavilion facets
  • 40%-41.2% is optimal for the reflection of light
• Culet size
  • ideally, this should be "none", meaning it is pointed rather than faceted
  • a faceted culet can appear as a dark spot when viewing the diamond from above
• Girdle size
  • a very thin girdle is susceptible to being chipped, but a very thick girdle adds to the carat weight (and cost) without making the diamond look bigger from above
  • ideally, the girdle should be in the range from Thin to Slightly Thick

Length-to-width ratio

Although not stated on the diamond certificate, the length-to-width (L/W) ratio is easily calculated by dividing the maximum diameter measurement by the minimum diameter measurement. In a perfectly round diamond, the minimum and maximum diameter measurements are the same, making the L/W ratio equal to 1. The larger the L/W ratio, the more oval in shape the diamond is, and so for a round diamond, you want it to be as close to 1 as possible. Anything up to about 1.02 looks perfectly round to the naked eye.

Heart & Arrows

Some round brilliant cut diamonds are marketed with a Hearts & Arrows grade. This refers to a specific pattern of light and dark that is visible in diamonds that have been cut with extreme precision and symmetry. While this marks these stones out for the exceptional skill required to cut them with such precision, there is no consensus as to whether the diamonds are any more attractive as a result, and there is often a significant price premium attached to them. Our recommendation is to judge for yourself whether you feel a Hearts & Arrows diamond looks superior and, if so, whether it is worth any premium being charged for this.

The best settings for a round brilliant cut diamond

The round brilliant cut diamond is highly versatile and can look stunning in a variety of settings. We've set out below some of the most popular round brilliant cut engagement ring styles, but it is important to ensure that you choose a setting that takes into account the wearer's own style and tastes.

Round Solitaire Engagement Rings

For those looking for a timeless, elegant look, classic solitaire settings highlight the diamond without any distractions, making it the focal point.

Round cut engagement rings with diamond shoulders

For those who want a dazzling, wow factor, melée diamonds set on the band add extra sparkle.

Round halo engagement rings

For those looking to maximise perceived size and sparkle, a halo of smaller diamonds encircles the centre diamond, making it appear larger and more brilliant.

Round Diamond Trilogy Rings

For those who appreciate symbolic or sentimental value, two smaller diamonds or gemstones flank the main stone, representing the past, present, and future.

The history and evolution of round diamonds

The round brilliant cut diamond has evolved over several centuries, with incremental improvements aimed at maximising the gem's brilliance and fire. Through continuous refinement and aided by technological advancements, the round brilliant cut diamond has reached a level of sophistication that maximises its natural beauty, making it the most popular and sought-after diamond cut today.

Before the 17th century

The concept of diamond cutting dates back to the Middle Ages, but the early cuts were rudimentary and far less sophisticated than today's techniques.

Old Mine Cut and Old European Cut

Before the round brilliant cut, these were popular cuts that shared some similarities with the modern round brilliant cut, like a rounded shape. However, they had fewer facets and did not maximise brilliance as effectively. 

The Old Mine Cut, with its slightly squarish, cushion-like shape, dates back to the late 17th century. It gained significant popularity during the 18th and 19th centuries. This cut is characterised by its rounded corners, high crown, deep pavilion, and large culet. The Old Mine Cut diamonds were hand-cut, which is why they have such a unique and individualised appearance. The cut was most prevalent before the development of modern diamond-cutting techniques, and it can often be found in Georgian (1714-1837) and Victorian-era (1837-1901) jewellery. The Old Mine Cut gradually evolved into the Old European Cut as cutting techniques improved.

The Old European Cut diamond was developed during the late 19th and early 20th centuries, roughly from the 1890s to the 1930s. It is considered a precursor to the modern round brilliant cut. The Old European Cut has a very distinct appearance, characterised by a high crown, small table, and open culet. With the advancement in diamond cutting techniques and a better understanding of light dynamics, the cut eventually gave way to the more brilliant and refined round brilliant cut that is popular today.

Marcel Tolkowsky

In 1919, Marcel Tolkowsky, an engineer and a third-generation diamond cutter, published his thesis "Diamond Design," where he laid down the theoretical principles for cutting a diamond to maximise its brilliance and fire. He is often credited with the development of the modern round brilliant cut.

Introduction of Modern Tools

As technology improved, particularly in the late 20th century, more precise cutting tools allowed for even greater exactness in creating facets and angles, improving upon Tolkowsky's original design. Today, the majority of the round brilliant cuts graded by GIA fit into the top two grades: Very Good and Excellent.

Computational Optimisation

In recent decades, computer models have been employed to further optimise the cut for even greater light performance, resulting in what we now know as the "super ideal" cuts that go beyond Tolkowsky's original specifications.

Round diamond FAQs

Summary

The round brilliant cut diamond, with its unparalleled brilliance and timeless appeal, makes for an exceptional choice for anyone seeking a classic yet spectacular gemstone. With its history deeply rooted in both art and science, it stands as a testament to human ingenuity in unlocking the full potential of nature's treasures.

By understanding its anatomy, the importance of cut quality, and how it interacts with other factors like carat, colour, and clarity, you can make an informed choice that will bring a lifetime of joy and perhaps even become a cherished heirloom for generations to come.

Whether you're celebrating an engagement, an anniversary, or simply indulging in a luxurious gift, a round brilliant cut diamond is more than a purchase; it's an investment in enduring beauty and lasting memories. Please feel free to get in touch if you have any questions. We'd be delighted to help!