January 28th 2022
The Round Cut Diamond Guide
By Devin Jones
StoneAlgo's top tips for maximizing your budget and picking the perfect round cut diamond.
Round diamonds are by far the most popular diamond shape for engagement ring diamonds and the vast majority of diamond inventory is round shaped. In fact, StoneAlgo was originally designed to help our co-founder Devin Jones pick out a round diamond, so this is really our home turf. Click here to run a quick search of all round cut inventory from the top online jewelers.
The popularity of round diamonds comes at a price: round diamonds are the most expensive shape across the board. You should target an excellent or ideal cut GIA or AGS certified round diamond with excellent polish and symmetry (GIA and AGS have the highest/most consistent standards of the rating agencies we’ve evaluated). For color and clarity – if you’re choosing a platinum or white gold setting we recommend J color or better, for a gold setting K color or better, and for both we recommend at least SI1 clarity (but prefer VS2 if you’re going for a larger diamond). The most important aspect to consider in a round diamond is the cut quality which directly effects the light performance (fire, brilliance, scintillation) of the diamond and is an affordable upgrade (it doesn’t cost significantly more to buy a superior cut quality diamond).
Round Diamond Prices
Round is the most popular diamond shape, and also the most expensive as we noted above. Below is a chart depicting the price (y-axis) trend for a large population of H VS2 round diamonds (GIA certified triple excellent). The price trend was solved using a simple regression on a the data set – our pricing algorithms use a more sophisticated machine learning algorithm than this model and incorporates several additional factors including each diamond’s specific dimensions. However, for studying simple price trend the below will paint a simpler picture. As you can see, a 1 carat H VS2 round diamond costs approximately $7,000 while a 2 carat costs approximately $20,000. This exponential growth in price (the price more than doubled when the size doubled) is due to the diamond pricing metric that most jewelers cite when discussing value: price per carat. Every diamond shape exhibits a similar exponential price trend as carat size grows larger. This is summed up by saying, “the price per carat increases as the carat size increases”.
Ideal Dimensions, Proportions, and Angles for a Round Cut Diamond
StoneAlgo always recommends you consider GIA excellent or AGS ideal cut round diamonds to ensure your diamond exhibits optimal light performance. Beyond simply recommending excellent/ideal, we conduct an algorithmic study of diamond dimensions to determine the light performance capability of each diamond in our database. For example, some diamonds are barely excellent cut, while others are examples of cutting perfection. As shown below, the difference can be significant between varying degrees of cut quality.
Both of the above diamonds are graded GIA Excellent but diamond on the left has a low diamond cut score while the diamond on the right has a very high cut score.
Excellent is the highest cut grade awarded by the GIA, while ideal is the highest grade awarded by the AGS for diamond cut quality. Some online jewelers will generalize both terms as “ideal” – but when it comes to GIA certified diamonds you can chalk this up as “excellent” cut. So which is superior and why? Well, the AGS has higher cut specifications, meaning that the “lowest” ideal cut AGS diamond will typically be better than the “lowest” GIA excellent cut diamond.
Here at StoneAlgo we judge diamonds on the their dimensions in order to calculate our cut score, and round diamonds are pretty tough to judge since there are so many moving parts. Basically, there is a balancing act that goes on between the girdle thickness, table %, depth %, pavilion angle, and crown angle. For a given depth, table, and girdle thickness there are an ideal set of angles that will return light most optimally. Our Cut Score was designed to simplify this process and guarantee that we find the most well balanced diamonds. Here is a rough idea of the ideal dimensions for a round cut diamond:
- Ideal Table Percentage: 54% - 58%
- Ideal Depth Percentage: 61% - 62.5%
- Ideal Girdle Thickness: Thin - Slightly Thick
- Ideal Pavilion Angle: 40.65 - 40.9 degrees
- Ideal Crown Angle: 33.8 - 35.2 degrees
- Culet: None or Pointed
As we noted, the ideal dimensions change to balance one another out. For example, a higher pavilion angle will be balanced by a lower crown angle and vice versa. These angles directly affect the depth percentage of the diamond as well. If you want to know exactly how well cut a round diamond is we recommend you use our cut score to ensure you aren’t making a mistake based on incomplete information. If you’d like to dig more into how the ideal dimensions work we recommend checking out the cutting chart guidelines from the AGS here or by reading the Diamond Design book by Marcel Tolkowsky here.
Round Diamond Color Recommendations
When it comes to color and clarity, round diamonds finally offer an opportunity to save a little money versus the fancy cuts. That’s because round diamonds do a better job of hiding both color and clarity than their fancier brethren. So, what color/clarity should you purchase? Well, it depends on a few things. Firstly, are you trying to maximize carat size or are you far more concerned with color? Our recommendation is to find a happy medium between the two. For larger diamonds, you’ll want to go with a slightly better color/clarity grade combination than for smaller diamonds because the color and clarity become more visible as the diamond size increases.
In addition, the type of metal you choose for your setting will directly impact the appearance of the diamond. It may sound odd, but if you have a more “colorful” setting (think gold/rose gold) you should pick a more “colorful” diamond as well – such as a K color instead of an I color. For this reason we recommend the below guidelines to help get the best balance of carat size and color for your budget. The best color for a round diamond on a platinum or white gold setting ranges from H to J color grades (GIA grading scale).
However, if you are choosing a yellow or rose gold setting, we would move down to a K color for round diamonds because the yellow gold will help hide some of the diamonds natural color. Below are examples of an I color in platinum and a K color in yellow gold taken from James Allen. Click to see more images like these by selecting “recently purchased” images below the main diamond image here for platinum or here for gold.
Round Diamond Clarity Recommendations
Clarity is the toughest aspect of a diamond to judge simply based on a diamond’s grading certificate alone. Two diamonds of the exact same size and clarity grade can look totally different depending on the types of inclusions in the diamond and where they are located. Generally speaking, these differences in appearance are not relevant in the better clarity grades like FL, IF, VVS1, VVS2, VS1, and even VS2 clarity – though occasionally a VS2 clarity will not be as eye clean as you may expect it to be. Eye clean diamonds are diamonds that don't have any visible flaws without the aid of magnification. On the flip side, SI1 diamonds can appear totally eye clean when they have inclusions in a certain location where it is less visible. Eye clean means that a diamond looks flawless without the aid of magnification from a jewelers loupe or microscopic lens. Here’s an example of a 1 carat F color SI1 clarity diamond that doesn’t appear to be eye clean (notice the black speckles across the top right corner).
Compare the above image to the below picture of another 1 carat F color (but this time eye clean) SI1 – the diamond received an SI1 grade due to some internal graining not shown as you can see by clicking the diamond and viewing its GIA certificate. This type of inclusion is tough to see even under magnification and is a good example of why eye clean SI1 diamonds are the most sought after when buyers are trying to stretch their engagement ring budget. We would note, however, that eye clean SI1’s typically cost about the same as VS2 clarity diamonds and sometimes even more – don’t simply trust that due to the grade you are getting a good deal!
The worst place for an inclusion is in the center/top of the diamond because this is the most visible location. The below is a diamond that has a large black inclusion right in the top center area – more visible in the video on James Allen’s site but you can clearly make it out below as well. If an inclusion appears off to the side of the diamond it can sometimes be covered by one of the prongs on the ring setting and any skilled jeweler will look for the opportunity to do so when setting a stone. We wouldn’t recommend you pay more just to go up in clarity if you can’t see a real, physical difference when you look at the diamond at it’s normal size (most images/videos online are zoomed in at least 10x – you can zoom out to get a better idea of how the diamond will look on a hand).
Round Diamond Fluorescence Recommendations
Diamond fluorescence produces a glowing property in some diamonds when they are exposed to ultraviolet light. To give an example of how prevalent fluorescence is in the diamond market, this affect is present in about 35% of the diamond inventory available at Blue Nile currently. Fluorescence is not necessarily a good thing or a bad thing, but like most things in life it’s good in moderation or in certain situations. For example, if you are buying a colorless diamond (D – E), then fluorescence may not be a good thing. But for diamonds graded I – K and lower, faint or even medium fluorescence can actually help the diamond appear whiter. Strong fluorescence is typically not recommended as it can make the diamond look hazy – so we’d recommend you stick with faint or none to be safe. You may have seen the designation negligible fluorescence which is the AGS grading equivalent to the GIA’s “none” qualification – representing diamonds that do not exhibit fluorescent properties.
Drop us a Line if you need further assistance!
That concludes our general recommendation for round diamonds. There are a lot of moving parts but generally speaking we try to maximize carat size for our given budget by sticking to the lower end of the color ranges we specified and searching for VS2 or better diamonds. SI1 diamonds are great if they’re eye clean, but they are a pain to find and often end up costing just as much as a VS2 diamond would. If you need one on one advice, we offer free support to all our users – simply drop us a line at email@example.com