How important is the HCA Score of a diamond? HCA Excellent vs HCA Very Good Scintillation

Thanks Todd. I think these are great options – I appreciate your help. [referencing a prior email message] I am working on contacting Brian Gavin and High Performance Diamonds. One thing I noticed is the Crafted by Infinity Diamond by HPD scored a very good for scintillation on the HCA, compared to the Brian Gavin diamonds that both scored excellent. Is this important? I am willing to pay for a superior cut diamond, is a scintillation of excellent vs. very good noticeable? ~ Kevin M.

Background: This blog post is based upon an ongoing conversation which Kevin and I have been having via email, in which we’ve been discussing several options which I provided him with from the inventory of Brian Gavin Diamonds and High Performance Diamonds, per an inquiry which he submitted to me via my free Personal Diamond Shopper Service, hence the reference to “these are great options” in the paragraph provided above. I thought that the information provided in my reply to Kevin would be helpful to other people shopping for a diamond online, this is the content of that email adapted for a blog post:

Here’s the problem with the Holloway Cut Adviser (HCA) Kevin, it’s designed to be an “elimination” tool and most people tend to use it as a “selection” tool… Ask Garry Holloway himself and he should tell you the same thing. Let’s back up a step and learn how to use the Holloway Cut Adviser (HCA) as the diamond elimination tool which it is designed to be.

How to use the Holloway Cut Adviser:

Let’s begin by entering the measurements for the 1.17 carat, I-color, SI-1 clarity, Crafted by Infinity Diamond from High Performance Diamonds into the Holloway Cut Adviser as follows:

Total depth: 61.2
Table diameter: 56.4
Crown angle 34.2 degrees
Pavilion angle: 40.9 degrees
Girdle edge: thin to medium, faceted
Culet: pointed

Enter these measurements into the HCA, as pictured in the results which appear above, and it indicates that the diamond has Excellent Light Return, Excellent Fire, Very Good Scintillation and Very Good Spread, however note that the sentence provided at the top of the score indicate that the result is based on diamond which is symmetrical with a medium girdle and very good polish. Since the girdle edge of the 1.17 carat, I-color, SI-1 clarity, Crafted by Infinity Diamond from High Performance Diamonds is thin to medium, we already have a variable which should be taken into account.

HCA Results for CBI Diamond AGS #104066143004 with Pavilion Angle adjusted one tenth of a degreeNow drop the pavilion angle to 40.8 degrees as can be verified by comparing the top line of the graphic pictured to the left with the one which appears at the top of this article where 40.9° has been changed to 40.8° and the results of the Holloway Cut Adviser (HCA) for this diamond change from EX | EX | VG | VG to EX | EX | EX | VG.

Do you really believe that a decrease of one tenth of a degree is going to make that much of a difference?


[Pause for Dramatic Effect]

Before you answer, I’ll openly admit that the question might seem a bit confusing since throughout this web site I clearly state that my preferred range for crown angle is between 34.3 – 34.9 degrees, and that I prefer a range between 40.6 – 40.9 degrees for the pavilion angle, which is likely to create the belief that a difference of a tenth of a degree must be important… Otherwise why wouldn’t my preferred range of proportions be tighter or broader?

For instance, why don’t I open up the range for crown angle to 34.0 to 35.0 degrees and the pavilion angle from say 40.3 to 41.2 degrees? Believe it or not, the actual range for these measurements could actually be much broader and a round brilliant cut diamond with a 56.4% table like the 1.17 carat, I-color, SI-1 clarity, Crafted by Infinity Diamond from High Performance Diamonds might still qualify for the zero ideal cut proportions rating which is not to say that it would score zero ideal for light performance, that is based on an entirely different scientific process known as Angular Spectrum Evaluation Technology (ASET) but it might receive a rating of AGS Ideal-0 for proportions and it certainly could get an overall cut rating of GIA Excellent from the GIA Laboratory which does not use ASET as part of their grading process.

So here’s the kicker…

The measurements for crown and pavilion angle stated on diamond grading reports are the average measurements for each section, based upon eight individual measurements… So that average of 40.9 degrees could be the result of something tight such as a low of 40.6 degrees and a high of 41.2 degrees, or it could be the result of a swing of a full degree ~ which is something I see a lot with diamonds graded by the GIA, because the diamond cutters know that they can get away with it, but also know that diamonds cut like that will not survive the Angular Spectrum Evaluation Technology (ASET) incorporated as part of the AGSL’s Proprietary Light Performance grading platform.

To make matters more interesting, the measurements stated on diamond grading reports are also rounded off, so if the averaged average pavilion angle of this diamond (let that concept sink in for a minute) measured 40.85 degrees, it got bounced up to 40.9 degrees, but if the averaged average pavilion angle measurement was 40.84 degrees, then it would have been dropped down to 40.8 degrees.  The same principle holds true for the average crown angle stated on the diamond grading report.

Now consider the fact that the HCA only takes the average of the average measurements into account when it spits out those ratings for brilliance, dispersion, and scintillation, and tell me, how accurate can the HCA score of a diamond possibly be?

This was the topic of a rather heated debate between Garry Holloway and I when he first introduced this tool, I urged him to add additional fields that would account for the high and low measurements for the crown and pavilion sections, however he correctly pointed out that the majority of diamond vendors do not provide this type of information and thus it would render the Holloway Cut Adviser useless for the majority of online diamond buyers and at the time it was introduced, I believe that it was a critically important tool which enabled people to narrow down the list of possibilities as they shopped for a diamond online.

Interpreting the Results of Sarin Proportions Analysis:

Thankfully both Brian Gavin Diamonds and High Performance Diamonds provide the results of Sarin computerized proportions analysis on their diamond details pages, this is the same technology which is used to measure diamonds by both the AGS & GIA gemological laboratories.

The data provided by a detailed Sarin scan can be confusing to customers, because they don’t understand the intricacies of diamond cutting and believe that there should be perfect symmetry of measurements all the way around the diamond, and the truth is that a certain amount of variance will result in a higher amount of contrast and sparkle, but too much will cause too much contrast and result in the upper girdle facets along the outer edges of the diamond to become too dark and this will make the diamond appear to be smaller than it actually is, an effect which caused the AGSL to tighten the parameters for their overall cut grade back in 2005 because the diamond cutters had figured out how to cheat the system.

The HCA is intended to be a Diamond Elimination Tool:

At the end of the day, the results of the Holloway Cut Adviser should only be used for the intended purpose of eliminating diamonds which clearly lack visual performance and exhibit less desirable levels of light return, but not as a diamond selection tool.

HCA results are based on measurements which are rounded offNotice that if you drift down to the bottom of the page for the HCA on Pricescope, that it alludes to the fact that the measurements for diamonds are frequently rounded off.


However most people either don’t read this reference, or don’t fully understand it because it is not explained in-depth. So the reason why I recommend that people search for round brilliant ideal cut diamonds which have a crown angle between 34.3 – 34.9 degrees and a pavilion angle between 40.6 – 40.9 degrees, while heavily promoting diamonds graded by the AGSL instead of those graded by the GIA, is because I know from experience that this range of average measurements tend to result in a higher volume of light return, with the understanding that the actual range for the crown and pavilion facets are going to be broader than the average stated on the diamond grading reports.

When the range is expanded much beyond the range that which I recommend for crown and pavilion angle, then the field of possibilities is expanded further, and the likelihood of people who read my blog being able to select a diamond with high visual performance online, decreases more and more with every step beyond the center range which I adhere to.

While a step of a tenth of a degree, one way or the other for crown or pavilion angle by itself, is not likely to create a distinct difference in the visual properties of the diamond, it does create subtle differences in the volume of light return and can affect the sparkle factor..

The Basic Geometry of Diamond Design:

Keep in mind that every diamond produced is essentially a puzzle, a geometric model which is designed to direct and redirect light around within the diamond in specific directions, which have the potential to increase light return and create sparkle in the form of brilliance, dispersion and scintillation.  I operate from the theory that a specific range for the crown angle and pavilion angle produce a relative balance of brilliance and dispersion, this theory is in line with the premise explained in Tolkowsky’s Diamond Design published in 1919 by diamond cutter Marcel Tolkowsky.

However there is also evidence that other combinations of crown and pavilion angle will produce similar volumes of light return and sparkle factor, such as a shallower crown angle offset by a slightly (read: SLIGHTLY) steeper pavilion angle, or a steeper crown angle offset by a shallower pavilion angle. The challenge is that the majority of diamonds cut to the outer edges of the parameters stated for the zero ideal cut proportions rating are most often cut for retention of carat weight, and thus they often lack optical symmetry and the contrast which results from higher levels of optical symmetry, and thus they tend to wash out when not being pumped full of light in the controlled atmosphere created by expensive jewelry store lighting.

25+ Years of Diamond Buying Experience:

Experienced diamond buyers like myself, have enough working knowledge of diamonds to be able to look at the combination of proportions stated on a diamond grading report, analyze the results of a Sarin | OGI | or Helium proportions scan showing the high and low range of facet measurements, and even take into account how the measurements for each crown and pavilion facet measurements offset each other (remember that a diamond is a 3D geometric puzzle) and analyze the images produced by photographing the diamond as seen through an ASET Scope, an Ideal Scope, and a Hearts & Arrows viewer, and get a pretty good feel for how the diamond is likely to look in terms of light return and visual performance… but the average consumer lacks the experience of looking at tens of thousands of diamonds under controlled lighting and being able to compare them side-by-side.

Thankfully you don’t have to become a virtual diamond expert in order to purchase a beautiful diamond online, because the information provided by the ASET scan which is provided on diamond grading reports issued by the AGS Laboratory evens the playing field… it picks up where the HCA falls short, by looking beyond the average proportions of a diamond and providing insight into the actual light return of the diamond. Thus the HCA can be used by consumers as a diamond elimination tool, as intended by its creator Garry Holloway, to narrow down the field of possibilities being considered online, and then the options which remain can be narrowed down further by comparing the measurements, and studying the images of the diamond as seen through the various reflector scopes.

Another factor to consider is that the parameters used by the Holloway Cut Adviser have not been adjusted since February 6, 2003 however both the GIA Laboratory and the AGS Laboratory adjusted the criteria for their diamond proportions grades in 2005, based on advanced research which involved an extensive amount of computer modeling and evaluation of actual diamonds.

One of the reasons why I refer people to Brian Gavin Diamonds and Crafted by Infinity via High Performance Diamonds so often, is because I am extremely familiar with their production quality and find it to be consistent. I have literally purchased thousands of diamonds from both companies over the years and have never [read: NEVER] rejected a single one of their diamonds for lack of light return or visual performance… I have rejected diamonds from Brian Gavin and Crafted by Infinity for clarity characteristics, however that is something which is easily addressed by carefully considering the information provided on the plotting diagram of the lab report and the clarity photographs these days.

About the Author Todd Gray

Todd Gray is a professional diamond buyer with 30+ years of trade experience. He loves to teach people how to buy diamonds that exhibit incredible light performance! In addition to writing for Nice Ice, Todd "ghost writes" blogs and educational content for other diamond sites. When Todd isn't chained to a desk, or consulting for the trade, he enjoys Freediving! (that's like scuba diving, but without air tanks)

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