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Foundation for the HCA

    Mr. Holloway happens to be one of the very first FireScope? owners who purchased his in the mid 80's when they were first made available for sale to the general public.  From noting and studying the primary angle combinations and relational table sizes (crown angles, pavilion angles and table size) that contribute to solid red/black reflector images, Garry has developed an easy way for consumers to get a virtual peek at which stones will and will not fare well through a reflector technology such as a FireScope, IdealScope or DiamXray viewer.  We believe Garry has the best interests of the consumer in mind by making this tool available to the public to use for free. I know a lot of time and effort has gone into putting it together and to make it available to the public for free can only be applauded. 

So, in essence ... a great HCA score on a diamond = a diamond that will get an excellent reflector image through one of the scopes/devices mentioned above and apparently a more beautiful diamond.  This is indeed the case in many instances but there are many other instances in which it is not which we will examine  more closely as we delve further into this study.

For a cursory understanding, looking at the 2 red reflector images side by side it shouldn't be too difficult to figure out which diamond should get the better HCA score.  A quick simple interpretation of red reflector images (a more exhaustive article on interpreting reflector images can be found here) is you want to see a good saturation of reds with a good distribution of blacks.  White = light leakage so the less of it the better.  The grading system starts at 0 and goes to 10 on what is supposed to be a linear scale.  0 being the best with 10 being the worst.  HCA Scores between 0-2 are deemed as the rarest and best.

HCA Score = 6.7	HCA Score = 1.5
Total depth: 65.1% Table: 63.2% Crown angles: 38.6° Pavilion angles: 41.7°	Total depth: 61.1% Table: 56.6% Crown angles: 34.9° Pavilion angles: 40.8°

So ... for the most part this is all well and good eh?  This assessment is accurate as we've personally confirmed it with a live analysis of the diamonds in question.  Here is a photograph taken of these 2 diamonds in simulated diffuse daylight (GIA DiamondDock?) which plainly demonstrates the optical characteristic of brightness.  The difference is quite easy to see regardless of the size difference between these 2 stones.

With this particular example the same is true regarding the optical characteristics as observed in spot lighting. 
Here is a photograph of the same 2 diamonds taken in a spot lighting environment.

To view a video of this comparison in diffuse daylight and spot lighting  for a more accurate assessment click this link.

How the HCA Works and What it Attempts to Predict

The preamble to the HCA states "The HCA estimates a round diamonds appeal based on its potential ...

• Light Return
• Fire
• Scintillation
• Spread

A diamond getting a score of 0-2 is deemed to be "Excellent" and while it does state that most people prefer stones scoring in the 1-2 range, we find that most consumers natural tendency is to get the best HCA score they possibly can.  As both a bricks and mortar store and an online retailer of diamonds many requests online come in for scores between 0-1 as these folks believe they are being even more selective.  You will soon learn this is not necessarily the case regarding HCA scores and why.

Using the HCA requires the user to get some basic information about the diamond they are considering.  Namely ...

• The total depth percentage.
• The table percentage.
• Average crown angles or crown height. (angle measurements are more accurate)
• Average pavilion angles or pavilion depth. (angle measurements are more accurate)
• Culet size.

Below is a screen shot taken off the PriceScope website of the HCA software tool showing the user where to input the data.

Prior to January 2006 you could not get this proportion information (primarily crown and pavilion angles) off of most lab reports and one would need a Sarin, OGI or Helium report which gave the measurements you needed to input into the HCA.  As of January 2006 all GIA Reports include crown and pavilion angle data as well as other information needed to get HCA scores.  While GIA does a small amount of rounding in their measurements it is generally not enough to impact the face up appearance of a diamond although it could dramatically impact the HCA score. (Note: The GIA Excellent Cut Grade does cover a certain variety of appearances which we further discuss in our article  on the subject).  Below is a chart showing diagrams on the various reports generated for diamonds wherein you can get the total depth, table size, average crown angles, & average pavilion angles needed to get an HCA score.  On each of these diagrams you can find total depth %, table %, crown angles, pavilion angles and culet information.

Portion of New GIA Report showing proportion data
Portion of an AGS Report showing proportion data
One Sarin Report showing proportion data
Portion of OGI Report
Portion of Helium Report

Strengths of the HCA

On the surface the HCA makes a lot of sense because there are many instances where diamonds that have great reflector images are indeed beautiful looking diamonds.   The 2 example diamonds we showed above is a classic case in argument for the HCA.  Garry has done a great job of establishing cut off points on the HCA for when leakage under the table becomes present in diamonds too.  For sake of example we'll be using live stones and also simulated diamonds and graphics in DiamCalc to demonstrate this.

Many shallow angled combinations, while not making GIA Ex or AGS ideal Grade are bright and fiery diamonds.

Recently we acquired the diamond below on the left which is a 1.01ct I VS2 which has a GIA Cut Grade of "Very Good" because of a few reasons (very thin girdle, good symmetry are two reasons), but the one of note being a proportion set which contribute to a decrease in optics in the GIA system.  When we ran the diamond through AGS' official Performance Grading Software it scores an overall "1" for Light Performance taking a hit on the "Tilt" factor.  While this diamond does not make ideal grade for Light Performance in both GIA and AGS Cut Grading systems, it scores a favorable 1.5 on the HCA.

Here is a photograph under the GIA DiamondDock of this shallow angled combination alongside another diamond which also gets the GIA "Very Good" grade which has steeper/deeper angles.  Two varieties of GIA Very Good's.   In our observations the shallow angled stone on the left has the brighter appearance as we can observe greater brightness and contrast in it.  The HCA scores for the following 2 stones are beneath.

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1.01ct I VS2 shallow angled GIA VG	1.03ct H SI2 steep angled GIA VG
GIA FacetWare Results	GIA FacetWare Results

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The guarantee of solid or mostly solid red/black reflector images

Many ideal cut diamonds on the market today have crown angles in the 34.x° to 35.x° (although there are quite a few proportion combinations that produce ideal optics).; When you couple those crown angles with pavilion angles at or over 41° leakage faintly begins to occur under the table.; Here's an example generated within the DiamCalc software showing it.; The diamond on the far left; has ideal proportions and makes top grade in any system or technology.; In the center stone we adjusted the pavilion angles from 40.8° to 41° and you can note more light pink as opposed to dark red under the table facet and the stone on the far right is beginning to get whitish under the table with a 41.1° pavilion angle; White = blatant leakage which generally occurs as angle relationships tend towards the steeper/deeper side.; Note the decrease in the; HCA scores as we adjust the pavilion angles from 40.8° to 41° to 41.1°.

Ideal cut diamond with a 61% depth, 56% table, 35° crown angles and a 40.8° pavilion angle.	Same as left but with a 41° pavilion angle.  Note how the reds get slightly more pale under the table facet.	35.1 crown angles, 41.1 pav angles & a 56% table.; Leakage is more apparent under the table as observed in red reflectors.
HCA = 1.5	HCA = 2.2	HCA = 3

So ... the HCA sees this faint leakage under the table of the 2nd stone and gives it a hit of 2.2 taking it out of the Excellent range and into the Very Good rating and the score decreases as it goes to the 3rd stone with slightly steeper angles and a little bit more leakage. 

Before we proceed to the limitations of the HCA, lets review it's strengths as covered thus far.

• An Excellent HCA score (0-2) = an excellent reflector image.
• An Excellent HCA score (0-2) = a diamond with minimal to no leakage via a reflector analysis, hence ...
• The HCA offers a critical exam of what angle combinations produce leakage in diamonds (particularly under the table).  Diamonds termed as "steep/deeps" (steep crown angles combined with steep pavilion angles produce leakage which the HCA easily and accurately identifies).
• Many diamonds with excellent reflector images are good looking stones.
• While the HCA favors non-AGS Ideal/GIA Ex shallow angled diamonds, it is this professional's opinion that these AGS Ex/GIA VG's are amongst the finest in their class (though not as bright as the best).
• Excellent for determining diamond appearance in spot lighting environments (fire) even if the diamond suffers in brightness.
• It's free! :)

The question that begs an answer then is "Do the seemingly best FireScope images always translate into the most beautifully cut diamonds?" And ... "Is this leakage which is visible under a reflector (foundation for HCA scoring), discernable to human eyes?"  If the answer to any of these questions are "no", then there are some apparent kinks in the system that consumers who consult this tool need to be made aware of as well.

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The Limitations of the HCA

When  you input these parameters into the Holloway Cut  Advisor here are the results.

If one were looking at a GIA Report or GIA FacetWare scan and inputting the numbers into the HCA this would turn off most consumers shopping for a superior cut quality.  After all, superior cut offers supreme beauty in diamonds. With ratings of Excellent, Very Good, Good, Fair and Poor nothing about this stone is Excellent according to the HCA. :-S  This  is sad because folks could be passing up a perfectly beautiful diamond based on this faulty information.

Our independent research has determined that with a 35 degree crown angle, leakage is not visible to human perception until the pavilion angles reach 41.4 degrees (November 06).

Even when we use the more precise measurements of the diamond via the Sarin analysis, the stone still scores over a 2.0.  :-S  The more precise measurements are under the column "Measured Value" in the graphic below.

Here are the more accurate HCA Results based on actual measurements of Sarin and not rounded with GIA's calculations.

1. The HCA attempts to predict potential GIA/AGS grades when it is possible that the diamond in question is perfectly fine, even with a score over 2.0.

   So one serious limitation of the HCA is that this HCA 2.3/2.1 (depending on which Report's numbers you input) happens to be as bright as as stones scoring under 2 (and  in some cases even brighter!).  Here is a graphic side by side of this 1.89ct G Si1 alongside a comparably sized 1.84ct F VS2 diamond which scores a 1.1 on the HCA.
    

   1.84ct F VS2 HCA 1.1	1.89ct G Si1 HCA 2.3/2.1

   To view a video of this comparison under diffuse lighting (for brightness) and spot lighting (for fire) click here.
   
   Using the same diamond as an example, let's look at limitation #3 of the HCA.
    
   

	a. The Table: Average of 4 table measurements.  (1 table facet) b. Crown Angles: Average of 8 crown angle measurements. (8 crown main facets)
	c. Average of 8 pavilion angle measurements. (8 pavilion main facets) 1 table + avg. of 8 crown mains + avg. of 8 pavilion mains --------------------------  = 17 facets.

The facets the HCA is not taking into account are

	d. 16 upper half facets.  e. 8 star facets
	f. 16 lower half facets. = 40 out of 57 facets the HCA is not taking into account!

Considering that the HCA only takes into account the averages of only 17 of 57 facets leads to HCA limitation #6.

1. The HCA does not take into account the variances of the facet measurements on which a diamond is cut.
   
   Examining the variances on a diamond is one way that shows the gemologist and the laymen the level of precision to which a diamond has been cut which can be examined numerically and based upon an accurate scan of the diamond being analyzed.  Take the diamond below.  A 1.34ct E VS2 which we examined.
   
   
   
   Note the variances for the measurements on each major facet set.  There is a .9° degree variance from minimum to maximum crown angles. A 1.3° variance from min to max on the pavilion angles and a 1.3° variance on all table measurements.  While this doesn't seem like a lot of variance on the surface, when you're talking about diamonds IT IS A LOT.
    

   Facets measured	Variance or difference from minimum to maximum
   crown angles (33.4 - 34.3)	.9o
   crown height (14.3 - 15.5)	1.2%
   pavilion angles (40.1 - 41.4)	1.3o
   pavilion depth (42.2 - 43.3)	1.1%
   table % (55.2 - 56.5)	1.3%
   culet off centered	.6%
   table off centered	1.0%

   Note this diamond for comparison.
   
   
   
    

   Facets measured	Variance from min to max
   crown angles (34.2 - 34.4o)	.2o
   crown height (15.0 - 15.3%)	.3%
   pavilion angles (40.7 - 40.9°)	.2o
   pavilion depth (42.6 - 42.9)	.3%
   table % (56.2 - 56.5&)	.3%
   culet off centered	.1%
   table off centered	.1%

   Note how tightly the variances are on this diamond when you take into account minimum/maximum measurements.  No comparison between the 2 when it comes to precision.  This diamond is cut to a MUCH tighter level of precision which the HCA does not tell you.  This limitation which can be examined in the numbers can also directly impact Limitation #7.
    
   

1. Since the HCA does not take 40 of 57 facets into account it does not take into account the cutting features of painting and digging which are minor facet deviations in the cutting of the upper and/or lower girdle facets.  How this impacts the optics of a diamond are notable enough to cause it to not make the GIA Excellent cut grade as well as the AGS Ideal cut grade. (Note: There are different thresholds regarding the amount of painting and digging allowable in each of the GIA and AGS cut grading systems for their top grade.)  The examples used below are diamonds that don't make either GIA Excellent Grade in both painting and digging while in AGS' system, the painted girdle diamond does make Ideal grade and is an example of digging the upper girdle facets. While there are various ways of being able to detect these features in girdle cutting we've included the results via the Helium Report for a quick evaluation. To learn more about painting and digging of the girdle facets we refer you to our more in-depth article here.
   
   In the chart below are the proportional measurements of each diamond, its HCA score then a photograph taken under simulated diffuse daylight alongside the diamond on the far right which gets an HCA of 2.1 (2.3 if using GIA's measurements). 
    

   	Dug out diamond (GIA Vg/AGS 1)	Painted girdle diamond (GIA VG/AGS 0)	Classic girdle cutting (GIA Ex/AGS 0)
   Proportions
   Helium paint/dug results
   HCA Results
   Photography			Each of the graphics on the left are showing this diamond (specs above) which is on the right hand side in each of the pictures.  A decrease in optics can be seen in both the dug out stone and the painted stone as exhibited by the presence of more dark areas than there are bright reflections of white light.  This is yet another instance where a GIA Ex/AGS Ideal with an HCA score >2 is brighter than these stones with HCA scores < 2.
   	To view a video of this comparison click here.	To view a video of this comparison click here.