Round Cut Diamond Guide

Last updated on December 24, 2022

Round is the world’s most popular diamond cut. In fact, round diamonds represent more than 70% of all diamonds sold globally because of their ability to optimize stone’s light performance. If you are interested in buying a diamond that has unparalleled brilliance, this round cut diamond guide is a must-read.

The History of Round Cut Diamonds

The history of the round cut, also referred to as the round brilliant cut, traces back to the middle of the 19th century, when diamond cutters began using more refined and complex techniques.

Lapidaries have been cutting diamonds with a round outline for centuries. However, it was not until the invention of the bruting machine in the early 1870s that a round diamond could be cut easily with a symmetrical outline. The first among more symmetrical cuts was the old European cut which also gave birth to the round cut.

The original round brilliant cut was created by a famous diamond cutter Marcel Tolkowsky in 1919. He published a work, “Diamond Design: A Study of the Reflection and Refraction and Refraction of Light in a Diamond”, which highlighted the ideal cut aspects of a round brilliant cut that allowed a colourless diamond to produce maximum fire, brilliance and scintillation.

The modern round brilliant cut is a modification of the original version. It features 56 symmetrical triangular and kite-shaped facets, a table facet and an optional culet facet, for a total of 57 or 58 facets. Since Tolkowsky’s influence, the round cut is the world’s most famous diamond cut, with a classic and timeless appearance used in every type of jewellery.

Round Cut Diamond Cut Quality

The round brilliant is the only cut lab entities grade for cut quality. The Gemological Institute of America (GIA) grades round brilliant diamonds on a five-point scale of Excellent, Very Good, Good, Fair and Poor, where Excellent cut grade diamonds provide the maximum brilliance.

The diamond cut grade is based on seven factors – brightness, fire and scintillation (how the diamond appears when viewed face-up), weight ratio and durability (how well it is designed to ensure durability and optimal weight), polish and symmetry (quality of craftsmanship). The grade is set by the lowest assessment the diamond receives for five out of the seven factors.

While the GIA cut grade is important, it is not the only aspect to consider when it comes to the cut, as there is a wide range of excellence within their grade. To evaluate the cut quality, you should also make sure the diamond is not too shallow or deep, as misalignment in diamond proportions causes a stone to lose its fire and brilliance.

According to Tolkowsky’s calculations, a round brilliant cut diamond should be cut to the following proportions: 34.5° crown angle, 40.75° pavilion angle, 59.3% total depth, 53% table diameter, 16.2% crown height and 43.1% pavilion depth. These calculations most likely correlate to an extremely thin girdle.

While Tolkowsky’s ideal cut standards serve as a global guideline for today’s diamonds, some countries have made their modifications, and there are at least six “ideal cut” standards used today in the diamond industry, but only three of them, including the one by Marcel Tolkowsky, are the most common.

The table below serves as a general guideline for evaluating the cut quality of round brilliant diamonds.

	EXCELLENT	VERY GOOD	GOOD	FAIR	POOR
TABLE %	53 - 58	52 - 53 or 58 - 60	51 or 61 - 64	50 or 65 - 69	< 50 or > 69
DEPTH %	59.0 - 62.3	58 - 58.9 or 62.4 - 63.5	57.5 - 57.9 or 63.6 - 64.1	56.5 - 57.4 or 64.2 - 65.0	< 56.5 or > 65.0
CROWN ANGLE	34.0 - 34.9	32.1 - 33.9 or 35.0 - 35.9	30.1 - 32.0 or 36.0 - 37.9	29.0 - 30.0 or 38.0 - 40.5	< 29.0 or > 40.5
PAVILION DEPTH	42.8 - 43.2	42.0 - 42.7 or 43.3 - 43.9	41.0 - 41.9 or 44.0 - 45.5	39.0 - 40.9 or 45.6 - 48.0	< 39 or > 48
GIRDLE	Thin to Slightly Thick	Very Thin to Slightly Thick	Very Thin to Thick	Very Thin to Very Thick	Extremely Thin to Extremely Thick
CULET	None	Very Small	Small	Medium	> Medium
L/W RATIO	1.00 - 1.01	1.00 - 1.01	1.00 - 1.01	1.02	> 1.02

Round Cut Diamond Colour

The colour of round cut diamonds is graded on a scale from D to Z, where D signifies a completely colourless stone and Z means an easily noticeable yellow or brown tint.

Since round brilliant diamonds reflect more light than any other diamond cut, they tend to mask yellowish tints in a stone. Moreover, smaller round diamonds, those weighing 0.50 carats and less, hide colour better than larger ones do. That is why, depending on the size of the diamond you are going to purchase, you may not need to buy a premium colourless stone, even if you want it to look white. While it is usually impossible to see the difference between two adjacent colour grades for an untrained eye, the price difference can be significant. In general, it is recommended to opt for a J colour or better to have a diamond that will appear colourless to the naked eye.

The chart below provides a general guideline for evaluating colour in the round brilliant cut diamonds.

SIZE	EXCELLENT	VERY GOOD	GOOD	FAIR	POOR
< .50 ct	D - G	H - I	J - K	L - M	> M
.51 - 1.0 ct	D - G	H - I	J - K	L - M	> M
1.0 - 2.0 ct	D - F	G - H	I - J	K - L	> L
> 2.0 ct	D - F	G	H	I - J	> J

Round Cut Diamond Clarity

The GIA grades the clarity of round cut diamonds on a scale from FL to I3, where FL means a flawless stone and I3 indicates a heavily included diamond.

Due to their facet arrangement and excellent light performance, round diamonds hide inclusions and blemishes fairly well. Moreover, smaller round brilliant cut diamonds hide imperfections better than larger ones do, which is why if you are going to purchase a smaller stone, you may opt for a lower clarity grade such as SI1 and SI2 while keeping an eye-clean appearance.

The chart below provides a general guideline for evaluating clarity in round cut diamonds based on the GIA clarity grading standards.

SIZE	EXCELLENT	VERY GOOD	GOOD	FAIR	POOR
< .50 ct	FL - VS2	SI1 - SI2	I1	I2	> I2
.51 - 1.0 ct	FL - VS1	VS2 - SI1	SI2	I1 - I2	> I2
1.0 - 2.0 ct	FL - VVS2	VS1 - VS2	SI1 - SI2	I1	> I1
> 2.0 ct	FL - VVS2	VS1 - VS2	SI1	SI2	> SI2

Featured image: DiamondGalaxy / Shutterstock