Perhaps this is the first time I'm bringing up the DiamCalc software results in this tutorial but we have been noting correlations between DiamCalc results and visual observation for a while now. Earlier last year (in 2005) when we did our review of the GIA Cut Grading System we noted a direct correlation between the various GIA Cut Grades and the Contrast metric in the DiamCalc software. Results that can also be seen virtually through the DiamCalc/Gem Advisor software "Office Lighting View".  Below is a demonstration of the comparison we'll be making with photography & video except here are the DiamCalc results.

 

DiamCalc Metric	1.89ct w/HCA >2	1.73ct w/HCA of 1.5
Office Lighting
DiamCalc Cut Quality Results (note the lower "dark zone" score. This is the red flag we see which correlates to the AGS Contrast metric)

Note when you compare the 1.89ct with the 1.73ct in the simulated "Office Lighting" you can note more areas of darkness and less bright reflections in the 1.73ct.  Here are those graphics scaled down. for a side by side comparison.  The diamond on the left is what makes GIA/AGS' top grade for Light Performance while the diamond on the right fails in both systems for top optics (and for good reason we believe).

Here is a photograph of the same 2 diamonds taken under the GIA DiamondDock simulated diffuse daylight view.  Can you pick out which diamond is which based on the graphics above?  I think  it'll be easy for ya. ;)  The comparison of the DiamCalc Office Lighting View and the DiamondDock simulated diffuse daylight view (live) we have found to have the closest corelations when it comes to real world observations of the optical characteristic of brightness.  At least these have been my observations.

Here's the same graphic blow up to show more detail.



To view a video comparison of these 2 diamonds under simulated diffuse daylight click here.

Note: While the shallow angled diamond above suffers in diffuse day lighting environments (both simulated and natural) the diamond does happen to excel in direct/spot lighting environments (exhibiting fire/scintillation characteristics).  In our professional opinion, for a diamond to be considered as having top optics or Ideal/Excellent light performance it must excel not only in spot lighting but also natural day lighting as well.
 

For a basic understanding of spread, I've provided in the graphic below is a great example of how total depth % in a round brilliant cut diamond impacts its face up appearance.  Both of the diamonds in the photograph below weigh exactly the same and are 1.01ct in weight.  The diamond on the left is what is considered a commercial diamond and has a total depth percentage of 64.6%.  The diamond on the right falls in the zenity of GIA Excellent/AGS Ideal and has a total depth percentage of 61.1%

1.01ct w/64.6% depth.	1.01ct w/61.1% depth
While there is a notable difference in the brightness between the 2 diamonds above, what we are noting  here is the visual size difference between the two diamonds. The graphic is blown up to show more detail.  Not only is the diamond on the right brighter but it's diameter and visual size is notably larger.  Ie. it has a better spread.  The graphic below is scaled down.
This is a computer generated graphic of the same 2 stones via DiamCalc.

	Same example as above. 64.6% next to 61.1%.  We can most definitely note the difference in size visually.
	This comparison is a diamond with a total depth of 60.3% next to one with 61.1% depth.  No notable difference  in size.
	A 60.3% depth alongside a diamond with 62.5% depth.  Perceptible but not easy.

In the trade certain professional's have what they consider to be their "cut offs" and we most certainly have tolerances we abide by regarding the quality of the diamonds we feature and very particular about the cut qualities we deem as worthy to make it into our inventory.  While we respect Garry's personal opinion on the Spread grading in the HCA it is important for the consumer to understand that this is yet another instance where the HCA reflects Garry's personal opinion/interpretation that does not agree with most professional's and more notably does not agree with GIA and AGS' considerations of this feature.  If there were strong merit to it I would not have bothered writing about it but since we get questions from consumers on it I thought it best to address it in this article, especially for those who are being particular about getting 4 Excellent's with an HCA score.

Conclusion: There are many instances when the HCA is accurate but there are also many instances when it isn't.  While the HCA is sensitive about eliminating angle combinations that produce leakage under the table (steep/deep angle combos) it doesn't discern or give accurate break points where leakage impacts actual "Total Visual Performance" (ie. when leakage is discernable to the human eyes). So there are angle combinations that take a hit on the HCA (some used in this article) for Total Visual Performance and the leakage is impossible to discern with the eyes and in some cases even hard to discern under reflector based technologies such as the IdealScope/DiamXray/ASET .
    On the flip side the HCA is not sensitive to shallow angled combinations but reflects Garry's personal preference towards these diamonds.  There are shallow angled combinations that are more attractive than others however when it comes to actual "Total Visual Performance" the HCA doesn't do that good of a job separating amongst these shallow angled combos and the actual break points where shallow angles actually do impact face up appearance.  This is particularly true regarding diamond optics in daylight environments which happen to be some of the most common viewing environments people find themselves in daily.
    When a diamond falls within the threshold of GIA Ex and AGS Ideal for "Light Performance" and the HCA gives it a score over 2.0 we are finding  the HCA to be in error each time.  As daily buyers and traders of some of the finest diamonds in the world, it's neat to know what the HCA score is (and  many of our diamonds score =< 2.0) however we never let it dictate our purchasing decisions and neither should it be for the end consumer.  When you have the tools and expertise of a professional's live analysis, this takes precedence over a prediction any day.  Considering the amount of instances where diamonds with scores > 2 are more beautiful than many combo's with diamonds that score < 2 we suggest the following procedure to the amateur seeking to use this tool in their purchasing decision if indeed they are seeking beautifully cut diamonds.

• Firstly seek out GIA graded "Excellent" cuts = > January 2006 or AGS Ideal Cuts accompanied with the "new" grading Reports that take "Light Performance" into account => June 2005.  AGS, at this time is issuing both older and newer Lab Reports upon the request of the manufacturer so if looking for AGS graded diamonds look for the ones with the "Light Performance" metric.
• If the GIA Ex or AGS Ideal scores under a 2.0 HCA no prob.
• If the GIA Ex or AGS Ideal scores over a 2.0 don't be quick to "weed it out".  There are angle combinations that score over a 2.0 that are just fine.
• On a personal note, we use the most stringent criteria for determining top optics in diamonds and 99% of our inventory falls within the zenith of GIA and AGS top grade for light performance.  If you're going to deviate away from either labs specs for their top grade we recommend seeing it alongside one that does especially if you're going to put heavy emphasis into a particular technology (HCA, Ideal-Scope, BrillianceScope, etc.).
  • For further study: About a year ago we got our hands on a diamond that was a GIA Excellent but scored a 3.8 on the HCA that most observers (90% out of 30 observers) preferred over a stone scoring a 1.0 on the HCA.  For more on that here is the article.

Best regards,
Jonathan Weingarten
Gemologist, Good Old Gold
The Ultimate Diamond Information Site

辛苦啦，good post!