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Please....help me decide!

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
I am trying to choose between two beautiful round brilliant stones. Both are the same price, XXX diamonds, GIA Cert-

Choice 1:
2.04 ct
E
SI1
No flourescence

Measurements:
8.30 x 8.34 x,4.95


Choice 2:
2.01 Ct
D
SI1
Slight flourescence

8.21 x 8.25 x 89

Do I sacrafice color in favor of a better spread and no flourescence?

Thank you!
 

tyty333

Super_Ideal_Rock
Premium
Joined
Dec 17, 2008
Messages
22,663
Can you post the info from the GIA reports...

table
depth
crown
pavilion

Are these on-line stones? Put them on hold and provide links if so.
 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
I'd also welcome any estimates of what a good price would be for either of these stones. Thank you so much!
 

Snowdrop13

Ideal_Rock
Premium
Joined
Aug 27, 2011
Messages
2,137
Hi @Beckywecky, regarding price, I did a quick search on the tool above (under the “Resources“ tab) and there are 22 GIA XXX D,E,F Si1 stones, 2-2.05ct. They range from $15-25 k so pretty broad.

I‘ve run the numbers of your 2 stones through the HCA (which is used as a selection tool for the quality of the cut) the score on the second one is 0.9 which means it is worth considering, the first is 5.9 so I’d reject that one.
 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
Hi @Beckywecky, regarding price, I did a quick search on the tool above (under the “Resources“ tab) and there are 22 GIA XXX D,E,F Si1 stones, 2-2.05ct. They range from $15-25 k so pretty broad.

I‘ve run the numbers of your 2 stones through the HCA (which is used as a selection tool for the quality of the cut) the score on the second one is 0.9 which means it is worth considering, the first is 5.9 so I’d reject that one.
That's so interesting to me. I was told these are great cuts (both XXX), especially the spread. Do you know why they didn't score that high?
 

Snowdrop13

Ideal_Rock
Premium
Joined
Aug 27, 2011
Messages
2,137
GIA XXX is a very broad category of cut.

Generally look for the following numbers,

table: 54-58
depth: 60-62.3
crown angle: 34-35.0
pavilion angle: 40.6-40.9 (sometimes 41.0)

then put the numbers into the HCA (Holloway Cut Advisor). If the stone scores more than 2, it would generally be rejected.

I’m guessing you are spending quite a lot of cash on this stone, if it’s from a store. It’s definitely worth spending a bit of time educating yourself and seeing what’s available. Presumably you want maximum sparkle? You may find you are able to drop in colour too, an F or G would still face up white, you might be surprised.

Are you fixed on buying from this jeweller? If you post your budget we could suggest other options.
 
Last edited:

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
Hi @Beckywecky, regarding price, I did a quick search on the tool above (under the “Resources“ tab) and there are 22 GIA XXX D,E,F Si1 stones, 2-2.05ct. They range from $15-25 k so pretty broad.

I‘ve run the numbers of your 2 stones through the HCA (which is used as a selection tool for the quality of the cut) the score on the second one is 0.9 which means it is worth considering, the first is 5.9 so I’d reject that one.
I've tried to use that tool, and I get this message:

You have been identified as a person in the trade.
You will need to use the paid business plans located here:
www.HollowayCutAdviser.com/hcabulk

Are you in the trade?

Thank you.
 

tyty333

Super_Ideal_Rock
Premium
Joined
Dec 17, 2008
Messages
22,663
You need to hit Report Concern on one of your post and ask Admin to get rid of your Trade designation (since you are not in the trade).

There may be a way to do it yourself but I couldnt see where (if anyone knows, please post the location).

Ditto @Snowdrop13 GIAxxx is very broad and contains stones that are not really all that well-cut.
 

natasha-cupcake

Brilliant_Rock
Premium
Joined
Nov 16, 2017
Messages
1,145
The diamonds you posted are know as "60/60" stones because the table and depth percentages are both around 60%. This type of diamond will reflect a lot of white light and therefore emphasize "brilliance". However, the brilliance comes at the expense of "fire", which is reduced in 60/60 diamonds compared to diamonds with smaller tables and lower table to depth ratios. Most people on this forum would argue for diamonds that have a better mix between brilliance and fire. To be fair, many people do prefer the look of a 60/60 stone. If you are not familiar with the difference between the two, here's a video that shows the two types of diamonds side by side, and an article about 60/60 diamonds. You can make an informed decision for yourself, if you have an understanding of this difference.


 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
The diamonds you posted are know as "60/60" stones because the table and depth percentages are both around 60%. This type of diamond will reflect a lot of white light and therefore emphasize "brilliance". However, the brilliance comes at the expense of "fire", which is reduced in 60/60 diamonds compared to diamonds with smaller tables and lower table to depth ratios. Most people on this forum would argue for diamonds that have a better mix between brilliance and fire. To be fair, many people do prefer the look of a 60/60 stone. If you are not familiar with the difference between the two, here's a video that shows the two types of diamonds side by side, and an article about 60/60 diamonds. You can make an informed decision for yourself, if you have an understanding of this difference.


Thank you, your response is extremely helpful!
 

jasper

Shiny_Rock
Joined
Sep 16, 2001
Messages
131
@Beckywecky -- You might not be sacrificing color by going with the E(no fluor) instead of the D(slight fluor). The GIA lab's procedures for grading color allow the fluorescence to affect the color grade.
 

jasper

Shiny_Rock
Joined
Sep 16, 2001
Messages
131
My guess is that these two stones' crown and pavillion angles complement each other very well -- even better than the lab reports say. Stone #1 gives up some more fire, to get extra brilliance.

The differences in spread between these stones, or between either of these stones and a stone of the same mass and 34°/40¾°/55⅓%/typical girdle proportions, are in the "you might notice it if the stones were side-by-side, or you might not notice" range. Sort of like how you might notice the difference in color between D and E if the stones were side-by-side, or you might not notice.
 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
My guess is that these two stones' crown and pavillion angles complement each other very well -- even better than the lab reports say. Stone #1 gives up some more fire, to get extra brilliance.

The differences in spread between these stones, or between either of these stones and a stone of the same mass and 34°/40¾°/55⅓%/typical girdle proportions, are in the "you might notice it if the stones were side-by-side, or you might not notice" range. Sort of like how you might notice the difference in color between D and E if the stones were side-by-side, or you might not notice.
It’s nice to hear your positive remarks. When I told the jeweler some of the previous remarks about the proportions, recommended to not by stone, he seemed shocked. So much conflicting information. I definitely don’t want a lifeless stone. Color/fire rank high to me, especially with this sort of investment.
 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
Also, I’ve researched that HCA tool that people here use, and it does not appear to have a lot of credibility. Thoughts?
 

seaurchin

Brilliant_Rock
Joined
Nov 2, 2012
Messages
1,474
A few general tips:

Cut is usually considered the most important C by far in a diamond's appearance, though as you see, it's a bit harder to figure out.

Brick and mortar jewelry stores often have higher prices (and much more limited inventory) than online diamond vendors. (With either, be sure to check out their reputations though).

You can get more bang for the buck by going down in color. Personally, I had a set of jewelers CZ stones for comparing diamond colors (I'm not a jeweler but just dabble a bit) and couldn't tell the difference between D and H. I could only detect a slight difference between D and I when the stones were turned upside down (and of course, these were all also not mounted). Some people are more color sensitive though.

The presence of fluorescence tends to bring the price down a little. I don't know why because I think a diamond that appears blue under a black light is very cool and prefer it over one that doesn't.
 
Last edited:

seaurchin

Brilliant_Rock
Joined
Nov 2, 2012
Messages
1,474
Also, I’ve researched that HCA tool that people here use, and it does not appear to have a lot of credibility. Thoughts?
I am not an expert but personally, I think of it like those medical BMI charts that indicate if a person is underweight, in the correct weight range for their height, obese or morbidly obese. They're a good rule of thumb but can't account for every possibility. For ex., someone could show as "obese" according to the BMI chart when they're actually a professional body builder, with that extra weight being muscle rather than fat. Just as some diamonds are exceptions to the rule. Some diamonds don't fit the stated guidelines but are beautiful anyway because there are many factors and not all can be accounted for with that one tool, for ex. some old cuts.

Wait... Are you in the trade?
 
Last edited:

jasper

Shiny_Rock
Joined
Sep 16, 2001
Messages
131
@Beckywecky -- I consider the HCA to be quite credible. That is because I understand how it was built, and its limitations. The HCA is based on a few people's systematic observations of real diamonds with similar shapes to the diamond being scored. The limitations have to do with using limited, rounded data to match up the diamonds, and the unavoidable fact that it is based on someone else's opinion. Thus, the HCA has a disclaimer suggesting that it be used as a filter, not as a be-all-and-end-all metric.

The HCA has four pieces, somewhat arbitrarily weighted: Brilliance, Fire, Scintillation, and Spread.

The Spread value is the result of a calculation. Its purpose is to remind the user (and the cutter and jeweller) that carat-weight is not the best way to measure the size of a diamond. It has the benefit of penalizing stones with thick girdles. Thicker girdles intercept some light that could have returned to the viewer. I try to ignore both carat weight and the HCA's spread metric. In my analyses, I find it simpler to compare stone sizes using a metric based on the stone's diameter, and then include the girdle thickness as a factor in optimizing the crown angle and table ratio.

The HCA's Fire and Scintillation scores are weighted equally. The HCA's Brilliance score is weighted equally to the total of the Fire and Scintillation scores. This makes sense: fire and scintillation are closely related phenomena. When choosing the pavillion angle of a stone (and thus the optimum crown angle), there is a tradeoff between brilliance and fire. In his analysis, Tolkowsky weighted brilliance and fire equally for purposes of making this trade-off, and for purposes of optimizing the crown angle. The HCA basically does the same thing, but has two subscores within the fire half of the scoring.

The HCA's scores for Brilliance, Fire, and Scintillation are based on database lookups. @Garry H (Cut Nut) and his staff looked at a huge number of stones that passed through their stores, and gave the stones scores in these three categories. Essentially, they recorded their personal opinions about these aspects of the diamonds' appearances. The database uses a few pieces of data to look up stones with similar shapes, and check what Garry and his staff thought of those similar stones.

Some major limitations of the HCA (and the AGS and GIA cut grading systems, as well) have to do with the parameters used to perform the database lookups.
  • Only a few metrics are used; they are the ones that common non-contact diamond scanners output circa the year 2000. There are a few reasons for this. First, Garry was limited by the data that the scanner in his store gave him. Second, as a tool published circa the year 2000, the HCA was limited to the data that customers had available (which was usually from the same kinds of machines). Third, the parameters chosen severely constrain the range of possible shapes for the remaining facets. Fourth, cutters are working within a limited range of Star:UG ratios and lower girdle facet lengths.
  • Unfortunately, many of the metrics provided by the scanning machines have limited accuracy, and/or are rounded off. For example, I guess that some of the measurements on your example stones are off by 0.3 - 0.5 percentage points or degrees. Similar errors were present in the data Garry used to calibrate the HCA, and were de facto acknowledged by the scanners. (Early 2000s scanners consistently reported culet widths of 0.5 - 1.0 percentage points on stones that did not have culets.)
  • The HCA buckets diamonds based on rounded off metrics. (The AGS and GIA cut grading systems do the same thing, perhaps with different rounding thresholds.)
  • The HCA does not take into account asymmetries. (This is part of why many PriceScope commenters like to look at ASET, IdealScope, or hearts-and-arrows photographs or renderings; these tools reveal optical effects from asymmetries.)
  • Different lower girdle facet lengths have a noticeable effect on scintillation. Unfortunately, the HCA does not take lower girdle facet length into account. And even if the HCA did, GIA reports provide overly rounded lower girdle facet length data.
  • The HCA only takes into account girdle thickness via the spread penalty.
  • The HCA does not take into account "indexing" of the upper girdle facets. Most diamonds do not have indexed upper girdle facets. Some high-end brands were "painting" the upper girdle facets at the time the HCA was developed, but Garry was not aware of this initially. Other cutters were "cheating" the upper girdle facets; those cutters tended to cut stones that did not do well on the HCA for other reasons. ("Painting" and "cheating" are terms for the direction of the "indexing". Stones with "painted" upper girdle facets are more likely to have edge-to-edge brilliance, at the expense of less fire from the edges of the stone. Stones with "cheated" upper girdle facets hide extra weight in the crown and girdle, at the expense of both fire and brilliance.)
  • The HCA does not take into account the star facet length.
Notice that the HCA was showing trade-offs between crown angles, pavillion angles, and table ratios years before the AGS and GIA cut grading systems were revised to include the trade-offs between crown angles and pavillion angles. For that matter, the HCA was showing these trade-offs before anyone noticed that Tolkowsky's logic provided a way to quantify this trade-off. And notice that the trade-off observed by the HCA is very similar to what was later found by other researchers. (Or in Bruce Harding's case, earlier.)
 

jasper

Shiny_Rock
Joined
Sep 16, 2001
Messages
131
@seaurchin -- You are right that some people are more color-sensitive than others.

Martin Haske ( @adamasgem ) developed and sold machines for measuring gem color and composition. According to his interpretation of a respected reference work, it is common for people to be able to distinguish the colors of (equally well-cut) diamonds that are two color grades apart, with 90% accuracy. Distinctions of diamonds that are one color grade apart are much harder/chancier. (Partly because diamond graders have similar difficulties, and sometimes make mistakes.)
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
Joined
Aug 15, 2000
Messages
15,823
@Beckywecky -- I consider the HCA to be quite credible. That is because I understand how it was built, and its limitations. The HCA is based on a few people's systematic observations of real diamonds with similar shapes to the diamond being scored. The limitations have to do with using limited, rounded data to match up the diamonds, and the unavoidable fact that it is based on someone else's opinion. Thus, the HCA has a disclaimer suggesting that it be used as a filter, not as a be-all-and-end-all metric.

The HCA has four pieces, somewhat arbitrarily weighted: Brilliance, Fire, Scintillation, and Spread.

The Spread value is the result of a calculation. Its purpose is to remind the user (and the cutter and jeweller) that carat-weight is not the best way to measure the size of a diamond. It has the benefit of penalizing stones with thick girdles. Thicker girdles intercept some light that could have returned to the viewer. I try to ignore both carat weight and the HCA's spread metric. In my analyses, I find it simpler to compare stone sizes using a metric based on the stone's diameter, and then include the girdle thickness as a factor in optimizing the crown angle and table ratio.

The HCA's Fire and Scintillation scores are weighted equally. The HCA's Brilliance score is weighted equally to the total of the Fire and Scintillation scores. This makes sense: fire and scintillation are closely related phenomena. When choosing the pavillion angle of a stone (and thus the optimum crown angle), there is a tradeoff between brilliance and fire. In his analysis, Tolkowsky weighted brilliance and fire equally for purposes of making this trade-off, and for purposes of optimizing the crown angle. The HCA basically does the same thing, but has two subscores within the fire half of the scoring.

The HCA's scores for Brilliance, Fire, and Scintillation are based on database lookups. @Garry H (Cut Nut) and his staff looked at a huge number of stones that passed through their stores, and gave the stones scores in these three categories. Essentially, they recorded their personal opinions about these aspects of the diamonds' appearances. The database uses a few pieces of data to look up stones with similar shapes, and check what Garry and his staff thought of those similar stones.

Some major limitations of the HCA (and the AGS and GIA cut grading systems, as well) have to do with the parameters used to perform the database lookups.
  • Only a few metrics are used; they are the ones that common non-contact diamond scanners output circa the year 2000. There are a few reasons for this. First, Garry was limited by the data that the scanner in his store gave him. Second, as a tool published circa the year 2000, the HCA was limited to the data that customers had available (which was usually from the same kinds of machines). Third, the parameters chosen severely constrain the range of possible shapes for the remaining facets. Fourth, cutters are working within a limited range of Star:UG ratios and lower girdle facet lengths.
  • Unfortunately, many of the metrics provided by the scanning machines have limited accuracy, and/or are rounded off. For example, I guess that some of the measurements on your example stones are off by 0.3 - 0.5 percentage points or degrees. Similar errors were present in the data Garry used to calibrate the HCA, and were de facto acknowledged by the scanners. (Early 2000s scanners consistently reported culet widths of 0.5 - 1.0 percentage points on stones that did not have culets.)
  • The HCA buckets diamonds based on rounded off metrics. (The AGS and GIA cut grading systems do the same thing, perhaps with different rounding thresholds.)
  • The HCA does not take into account asymmetries. (This is part of why many PriceScope commenters like to look at ASET, IdealScope, or hearts-and-arrows photographs or renderings; these tools reveal optical effects from asymmetries.)
  • Different lower girdle facet lengths have a noticeable effect on scintillation. Unfortunately, the HCA does not take lower girdle facet length into account. And even if the HCA did, GIA reports provide overly rounded lower girdle facet length data.
  • The HCA only takes into account girdle thickness via the spread penalty.
  • The HCA does not take into account "indexing" of the upper girdle facets. Most diamonds do not have indexed upper girdle facets. Some high-end brands were "painting" the upper girdle facets at the time the HCA was developed, but Garry was not aware of this initially. Other cutters were "cheating" the upper girdle facets; those cutters tended to cut stones that did not do well on the HCA for other reasons. ("Painting" and "cheating" are terms for the direction of the "indexing". Stones with "painted" upper girdle facets are more likely to have edge-to-edge brilliance, at the expense of less fire from the edges of the stone. Stones with "cheated" upper girdle facets hide extra weight in the crown and girdle, at the expense of both fire and brilliance.)
  • The HCA does not take into account the star facet length.
Notice that the HCA was showing trade-offs between crown angles, pavillion angles, and table ratios years before the AGS and GIA cut grading systems were revised to include the trade-offs between crown angles and pavillion angles. For that matter, the HCA was showing these trade-offs before anyone noticed that Tolkowsky's logic provided a way to quantify this trade-off. And notice that the trade-off observed by the HCA is very similar to what was later found by other researchers. (Or in Bruce Harding's case, earlier.)
WOW Jasper - what a wonderful breakdown and description of my little baby!
I would like to add that my patent for the looks like size appears to be in its final passing stage.

That adds to the actual dimensions a metric to account for peripheral brightness around the edges of a diamond there by making it appear larger in the case of, say, an EightStar diamond (with cheated girdle that I was aware of in 1999 thanks to Pete Yantzer AGS Lab director at the time).
Miss Bruce Harding :(


  • "The HCA does not take into account asymmetries. (This is part of why many PriceScope commenters like to look at ASET, IdealScope, or hearts-and-arrows photographs or renderings; these tools reveal optical effects from asymmetries.)"
HCA does now give a plus for H&A's and minus for lower grades of symmetry - it is not perfect but helps. And YES - use HCA to reject then the other tools (I make and sell) for selection.

  • "Different lower girdle facet lengths have a noticeable effect on scintillation. Unfortunately, the HCA does not take lower girdle facet length into account. And even if the HCA did, GIA reports provide overly rounded lower girdle facet length data."
I actually covered off lower and upper girdle facets in my patent but never had millions of dollars or diamonds to do the needful research.

My end game is covered by the new patent - to enter any shape diamonds .stl 3D file into HCA LL and give the apparent and relative to weight spread. That will not be a Cut Grade per se', but will help consumers overcome the huge problems with rejecting and selecting fancy shaped diamonds!
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
Joined
Aug 15, 2000
Messages
15,823
OH!!!
My patent was granted 2 months ago!


Jasper you may be interested to help get this baby on the road?
I need a ray tracing system to create images in a specific lighting scheme and tilt the diamonds - taking snaps of each titled image.
 

Garry H (Cut Nut)

Super_Ideal_Rock
Trade
Joined
Aug 15, 2000
Messages
15,823
@Garry H (Cut Nut) -- Thank you for the corrections and updates.

And a clarification: The EightStar has a painted girdle, not a cheated girdle.
Yes, and no. 8* did it for a special effect.
Cutters have always done it for weight retention.
Over done and it kills scintillation which is one of the (many) reasons 8* is no longer around. When GIA launched their grading system the downgraded some 8* diamonds from Excellent to Very Good for lack of contrast.
Digging is almost always bad (but can make small improvements on shallow diamonds).
Painting on slightly deeper diamonds is often a benefit.
 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
I am not an expert but personally, I think of it like those medical BMI charts that indicate if a person is underweight, in the correct weight range for their height, obese or morbidly obese. They're a good rule of thumb but can't account for every possibility. For ex., someone could show as "obese" according to the BMI chart when they're actually a professional body builder, with that extra weight being muscle rather than fat. Just as some diamonds are exceptions to the rule. Some diamonds don't fit the stated guidelines but are beautiful anyway because there are many factors and not all can be accounted for with that one tool, for ex. some old cuts.

Wait... Are you in the trade?
No, I’m not in the trade. Total novice consumer. I’m trying to get that trade designation removed. Thank you!
 

Beckywecky

Rough_Rock
Joined
Oct 16, 2020
Messages
27
@Beckywecky -- I consider the HCA to be quite credible. That is because I understand how it was built, and its limitations. The HCA is based on a few people's systematic observations of real diamonds with similar shapes to the diamond being scored. The limitations have to do with using limited, rounded data to match up the diamonds, and the unavoidable fact that it is based on someone else's opinion. Thus, the HCA has a disclaimer suggesting that it be used as a filter, not as a be-all-and-end-all metric.

The HCA has four pieces, somewhat arbitrarily weighted: Brilliance, Fire, Scintillation, and Spread.

The Spread value is the result of a calculation. Its purpose is to remind the user (and the cutter and jeweller) that carat-weight is not the best way to measure the size of a diamond. It has the benefit of penalizing stones with thick girdles. Thicker girdles intercept some light that could have returned to the viewer. I try to ignore both carat weight and the HCA's spread metric. In my analyses, I find it simpler to compare stone sizes using a metric based on the stone's diameter, and then include the girdle thickness as a factor in optimizing the crown angle and table ratio.

The HCA's Fire and Scintillation scores are weighted equally. The HCA's Brilliance score is weighted equally to the total of the Fire and Scintillation scores. This makes sense: fire and scintillation are closely related phenomena. When choosing the pavillion angle of a stone (and thus the optimum crown angle), there is a tradeoff between brilliance and fire. In his analysis, Tolkowsky weighted brilliance and fire equally for purposes of making this trade-off, and for purposes of optimizing the crown angle. The HCA basically does the same thing, but has two subscores within the fire half of the scoring.

The HCA's scores for Brilliance, Fire, and Scintillation are based on database lookups. @Garry H (Cut Nut) and his staff looked at a huge number of stones that passed through their stores, and gave the stones scores in these three categories. Essentially, they recorded their personal opinions about these aspects of the diamonds' appearances. The database uses a few pieces of data to look up stones with similar shapes, and check what Garry and his staff thought of those similar stones.

Some major limitations of the HCA (and the AGS and GIA cut grading systems, as well) have to do with the parameters used to perform the database lookups.
  • Only a few metrics are used; they are the ones that common non-contact diamond scanners output circa the year 2000. There are a few reasons for this. First, Garry was limited by the data that the scanner in his store gave him. Second, as a tool published circa the year 2000, the HCA was limited to the data that customers had available (which was usually from the same kinds of machines). Third, the parameters chosen severely constrain the range of possible shapes for the remaining facets. Fourth, cutters are working within a limited range of Star:UG ratios and lower girdle facet lengths.
  • Unfortunately, many of the metrics provided by the scanning machines have limited accuracy, and/or are rounded off. For example, I guess that some of the measurements on your example stones are off by 0.3 - 0.5 percentage points or degrees. Similar errors were present in the data Garry used to calibrate the HCA, and were de facto acknowledged by the scanners. (Early 2000s scanners consistently reported culet widths of 0.5 - 1.0 percentage points on stones that did not have culets.)
  • The HCA buckets diamonds based on rounded off metrics. (The AGS and GIA cut grading systems do the same thing, perhaps with different rounding thresholds.)
  • The HCA does not take into account asymmetries. (This is part of why many PriceScope commenters like to look at ASET, IdealScope, or hearts-and-arrows photographs or renderings; these tools reveal optical effects from asymmetries.)
  • Different lower girdle facet lengths have a noticeable effect on scintillation. Unfortunately, the HCA does not take lower girdle facet length into account. And even if the HCA did, GIA reports provide overly rounded lower girdle facet length data.
  • The HCA only takes into account girdle thickness via the spread penalty.
  • The HCA does not take into account "indexing" of the upper girdle facets. Most diamonds do not have indexed upper girdle facets. Some high-end brands were "painting" the upper girdle facets at the time the HCA was developed, but Garry was not aware of this initially. Other cutters were "cheating" the upper girdle facets; those cutters tended to cut stones that did not do well on the HCA for other reasons. ("Painting" and "cheating" are terms for the direction of the "indexing". Stones with "painted" upper girdle facets are more likely to have edge-to-edge brilliance, at the expense of less fire from the edges of the stone. Stones with "cheated" upper girdle facets hide extra weight in the crown and girdle, at the expense of both fire and brilliance.)
  • The HCA does not take into account the star facet length.
Notice that the HCA was showing trade-offs between crown angles, pavillion angles, and table ratios years before the AGS and GIA cut grading systems were revised to include the trade-offs between crown angles and pavillion angles. For that matter, the HCA was showing these trade-offs before anyone noticed that Tolkowsky's logic provided a way to quantify this trade-off. And notice that the trade-off observed by the HCA is very similar to what was later found by other researchers. (Or in Bruce Harding's case, earlier.)
Thank you for that comprehensive response. Extremely helpful!
 
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