Ready, Set, Go for Ideal Cut Proportions as determined by either the AGS Laboratory or by Sarin DiaMension Computerized Proportions Analysis with Polish and Symmetry ratings of AGS Ideal [ID] or GIA Excellent [EX] with a color between ‘D’ and ‘G’ for “no brainer” white and ‘H’ to ‘I’ to max out the size and the diamond will still face up white when set in white gold or platinum prongs. Most people are able to see color easily in diamonds that are J color and lower (L/M/N and so on…) because the tonal value of the diamond will appear “warmer” than diamonds in the D to I color range. There is no right and wrong when it comes to selecting a diamond color, it is just a matter of personal preference that is probably best considered in terms of an absence of color, some people prefer diamonds which are cooler in appearance and some people prefer diamonds which are warmer in hue and thus we suggest visiting a local jewelry store to compare diamonds of different colors to determine your personal preference for color. The reality is that diamond color has no effect on the actual visual performance of a diamond.
We feel that “AGS Ideal” and “GIA Excellent” with ideal cut proportions as verified by Sarin proportions analysis are essentially the same “highest grade possible” from the two laboratories since they seem to grade essentially the same (high standards and ethics with a knack for consistency). Although there are a myriad of combinations of angles and degrees which will produce optimal light return in a round brilliant ideal cut diamond, you can narrow the field of possibilities and dramatically improve your chances of successfully selecting a gorgeous diamond by keeping the proportions of the diamond within the center range of the spectrum for the ideal cut proportions rating. We consider the center range to be as follows:
Total depth between 59 – 61.8%
Table diameter between 53 – 58%
Crown angle between 34.3 – 34.9 degrees*
Pavilion angle between 40.6 – 40.9 degrees
Lower girdle facets between 75 – 78%**
Star facets between 45 – 50%***
Girdle thickness between thin and slightly thick
Culet: AGS pointed or GIA none
My preferred range of crown angle for round brilliant cut diamonds is quite narrow, and this is because it is likely to produce a virtual balance of brilliance (white sparkle) and dispersion (colored sparkle) when combined with a pavilion angle between 40.6 – 40.9 degrees. The idea here is to enable you to quickly and easily buy a top performing diamond by taking guesswork out of the equation.
This is not to say that other combinations of crown and pavilion depth won’t result in comparable light return, there certainly is the possibility for such diamonds to exist. We’re just trying to simplify your quest as much as possible by providing you with a tight range that almost always works. If you want to take the guess work out of buying a diamond for visual performance, then check out the options available within the Brian Gavin Signature Collection which have been hand selected for maximum visual performance. If you want to explore a broader range of proportions, then visit James Allen or check out the list of Certified Diamonds from Blue Nile.
The reason why I recommend keeping the Lower Girdle Facet (LGF) length between 75 – 78% is because this tends to produce “Broad Spectrum Sparkle” that is going to be larger in size, and which will look bolder, brighter, and more vivid, than the smaller pin-fire type sparkle that is created by lower girdle facets in the range of 80 – 82% for round brilliant cut diamonds with the proportions outlined above.
The problem with pin-fire type sparkle is that our human eyes tend to experience difficulty dispersing smaller flashes of white light into colored light / sparkle, and thus diamonds that exhibit pin-fire type sparkle are likely to appear to be more brilliant (white sparkle) than exhibit a virtual balance of brilliance and dispersion (colored sparkle). You might have noticed a gap between the range of 75 – 78% and 80 – 82% where 79% should be, that’s kind of the critical tipping point where the type of sparkle being created by the diamond might go either way, a good middle ground I suppose if you don’t know what type of sparkle you prefer.
However I’ll tell you that diamonds that exhibit pin-fire type sparkle tend to look best when exposed to candlelight or halogen lighting, which are both pin-fire type light sources, but they don’t look all that great, or perform very well, when viewed under diffused lighting conditions, which tends to be the type of lighting that most of us live and work under.
Note that the AGS Laboratory measures the length of the lower girdle facets by height, this is the same approach used by mathematical ray tracing software like DiamCalc by Octonus. However the GIA Gem Trade Laboratory measures the length of the lower girdle facets by radius, so that they can be measured using a table gauge, and the GIA rounds that measurement off to the nearest half a percent, whereas the AGSL provides the actual measurement without rounding it off. These differences between how the two gemological laboratories measure lower girdle facet length can create confusion, because the approach is clearly different, and thus so are the measurements!
Star facets are the small triangular shaped facets that are located on the crown section (top portion) of the diamond between the kite shaped bezel facets, along the edge of the table facet, they serve to round off the shoulders or lobes of the hearts that are created in super ideal cut diamonds like Brian Gavin Signature Hearts and Arrows diamonds when the facets are polished with an extreme degree of consistency and precision.
The optimum star facet length is between 45 – 50% which will create nice rounded shoulders on the hearts. However it is acceptable for the star facets to be between 40 – 58%. The length of the star facets is affected by the size of the table facet and the crown angle.
It is important to understand that the star facets are considered to be a minor facet group. While people tend to get caught up on specific measurements, I suggest simply focusing on the images. If the hearts pattern is formed consistently, the exact measurements are not important. If the hearts are irregular and there is heavy obstruction, then I’d suggest a different diamond.
If you are looking for a diamond that is absolutely “eye clean” from a top down perspective then look within the clarity range of VS2 to VVS1 with VVS1 being the high side of the range. Consider diamonds in the SI-1 clarity range if you’re trying to get the biggest bang for your buck and don’t mind an inclusion or two which “may be visible” without magnification. Consider diamonds graded as being VVS1 in clarity if you can’t sleep at night if the shoes in your closet aren’t facing the same direction. Don’t accept a diamond graded by any other laboratory other than the AGS, GIA, or HRD which happens to be the diamond grading laboratory for the Belgium High Diamond Council if you’re looking for the type of consistency that we are during the grading process.
Remember that a more in-depth explanation of the concepts touched on here can be found in the 5 Minute Version of Diamond Buying Success if you are looking for more information.